No, it depends on other aspects of their personalities. Perhaps Sp/So is more likely not to be attracted to crude humor, but I have known So/Sps with the raunchiest sense of humor.
I think this is a good thread because it can be easier than you think to mix the two up, particularly many So/Sps mistyped as So/Sx. Yes, there will the people who are very blatantly obviously their type, but the lines can blur among more balanced individuals, especially when the Sx instinct is misunderstood.
Sx seeks intensity of connection - be that to a thing, a person, an interest, a happy, etc. - and a sense of merging with those things. It's a simplification, but we'll go with that for now.
What you'll find in Sp is a very practical, stable, grounded sensibility about things.
I will use my roommate who I thought was an So/Sx for a long time as an example. She is also an ENTP so, by default because of the zaniness and brilliance that comes with being that type, it's not difficult to see why I thought she was an So/Sx. She is fun, has a certain whimsical quality, always cracking a joke of some sort (usually something absurd delivered in a dry way), engaging, very passionate about the things she likes (sewing, design, etc), and just a perky and enthusiastic person in general.
I used to think being Sx last made people lack a certain spark, fieriness, or liveliness about them, but that is not necessarily true. My roommate clearly has all those things. However, I noticed throughout the years living with her, she voiced out concerns of a more Sp nature a lot more than you'd expect to hear from an Sp-laster. She is more practical minded than me in many ways, very focused on her energy levels, responsibilities, comfort, any possible ailments. She has a very 'get it done' attitude. She will face palm at the crazy things that I do and is very mindful of safety.
Her Sp was coming out so strongly, but I was still confused about her type because it seemed weird to me at the time to push Sx to the last because I thought she was too vivacious for that - not to mention had a phase where she was obsessed with sex, and is still quite an avid fan. I remember before she was with her boyfriend, she was so sex-crazed, always wanted a boyfriend or fling or something, flirting with any guy she found attractive. She's stabilized a lot since then (thank her boyfriend for that!), but she'll crack sex jokes left and right, be quite crude in her explanations, etc.
Sx is *not* about sex. I mean, yes, it's called the sexual instinct for a reason: When aroused or engaged in sex, you are locked onto the person you're having sex, your concentration aimed fully at them (wanting, lusting after them), sometimes you can even feel a certain oneness - a deep and intense connection - with that person while in the act. That intensity found in sex translates to a certain 'lust' and intensity toward one's interests and passions in the sexual instinct.
While my roommate is fun and exciting and sweet, her Sp needs far overshadow her Sx, even if it is not nearly as non-existent as some people's inferior instincts. When I got to thinking about it, she has many friends and rarely brings up the level of closeness she has with them, even if it is there - it is not a priority. She is not as concerned about intensely connecting to something. I've seen her talk to many people, and to most (including those dearest to her) she will talk fairly the same way to all of them with just about the same energy level. She is pretty responsible and makes sure to stay on top of things.
So, yes, she is an So/Sp. I imagine an So/Sx would be more scattered as far as responsibilities go, but there is a certain depth to the connection they make with people, and they definitely *seek* it a lot more. You know how Sx is often described as missing something and looking for that special puzzle piece that will fit and complete it? It's like that, whether it is overtly stated or not, unless they have that thing that they are locked onto, they will seek it out, or if not so active, feel not quite right, 'incomplete' to a degree, unmoved. Sx wants that thing that lights its soul on fire and burn in it if they have to. XD Of course, as a secondary instinct it will be more muted than an Sx-dom.
I am an Sp-first so I understand how self-preservation concerns manifest. Things to do with sustaining the self will come out to the forefront - money, career, health, energy, comfort, grades, peace in its interpersonal relationships, anything to do with what could affect their security. Sp wants stability. If those things are taken care of, they worry less (and not every person will focus on all of those things; in most cases, it's just a couple.) They value things like moderation because gluttony or neglect can negatively affect those things, which will negatively affect their sense of security. They conserve their energy and resources, otherwise they might feel they will not have enough to fulfill their own needs.
Sx (by itself) does not give a damn about stability. In fact, it is ready to explode. It is the hot and cold, the really high end and the really low end, either holding in its energy when nothing is capturing its attention, or spending seemingly limitless amounts of energy on that thing that calls its attention. It wants to be a part of the thing that captivates it to open those floodgates. It doesn't just want to embrace it; it wants to sleep in its skin. It knows no boundaries once something has awakened it. Think of So as a sort of web or tree with little connections everywhere, ever growing and expanding in multiple directions; Sp a sort of walled or moated fortress, keeping things contained and safe, keeping danger at bay; and Sx as a blast from a laser beam or flamethrower, looking to obliterate any barriers between itself and its desired object, consuming it entirely in its wake.
As a decent method of pinpointing secondary instincts, looking to see what the inferior instinct may be can prove to be enlightening. In the case of my roommate, despite her fun-ness and wackiness, she just brought up Sp stuff waaayyy too much for it to be an Sp-laster, and it made me notice how little she factors intensely intimate connections into her decisions.
Anyways, I am rambling now and I know I'm not the best at explaining the instincts, but I hope this offered insight in differentiating between the types.
Sexual last feelers are usually quite humorless or at least don't like crude humour. Sexual lasts usually lack the "it" factor and can easily blend to the mass.