Yes, a Crinkle Crankle Serpentine wall that has no other job to do such as hold back soil and is only 15-18" high, has a decent foundation of compacted gravel and a foot deep concrete foundation and the units are mortared together will be able to support human weight...you won't need to double it at all unless you actually want people to be comfortable, have a place to set their plates of food and drinks. The difference in price for a crinkle crankle serpentine double wall and cap and a cmu garden wall not including the labor to construct a serpentine wall is huge. I'll go check out the difference but I'd still need more information; drainage, surcharge, how to get a 'cap' on a small serpentine wall only doubled (or spaced with fill inbetween). I am guessing a small serpentine wall with custom caps, doubled walls will infill will be at least 3X more expensive. You could hope for a 1' wide wall. Concrete masonry unit (CMU) is 2' by 2" by 2' in length each.
The simple answer is yes, that wall as long as it is mortared can be used for human butts and weight. Good luck anyone would chose to sit on that wall without a cap.
And even a serpentine wall done well will enact a straight line. Otherwise, you could go nuts with erratic radii, a curved line of build that is also erratic. Just do the foundation correctly, allow no surcharge or build up of soil on one side making this a free standing nonsensical crazy wall to go with your free form home.
I did a Dr. Suess landscape once. You know, Dr. Suess and the cartoons? We had huge cranes to fit huge weird trees into the landscape and built a kid's playhouse that looked like a hobbit home, and serpentine walls made of ledger stone. Very short walls, that came off the hobbit home. Lots and lots of money in Seattle.