You don't have to have two separate paths. You can create a single Blend from three triangles (small, large, small).
That's right. Thanks for the correction Jet
Last time I was making blends like this I had to mirror an odd shape that flips in the middle without transition and I had to cut the path in three parts with a small segment in the middle without a blend but in this case cutting is not needed. Blends can be made between any number shapes.
Nice Thanks guys, I apparently missed this tool somehow :/ Probably would have been useful before this!
Is there any way to lock the triangles' lower-most corner to the path, rather than using the central registration point?
If I understand it in the right way, you need to do (at least) five things (more than has been expressly mentioned already):
1) In Blend options, set Align to Path (to (at least more) corrctly follow the direction of the path),
2) For each blend stop, create two rectangles pointing the same way with the crucial corner coinciding, one of them being nofill/nostroke, and Group them (to have that corner centred in the blend),
3) Create the blend with the desired number of rectangles (sets), then Expand and Distribute the triangles (sets) evenly or however you want them (presuming you do not wish to have the larger ones closer than the smaller ones or maybe even overlapping), then create a fake 1 step blend (in order to have a (fake) blend to distribute, no need to worry the surplus will go away in 5),
4) Add Anchor Points (enough times to keep the evenness of the distribution, you will know if there are (not) enough),
5) After replacing the spine, Expand and delete the fake steps, thus ending with the desired number of triangles (you may delete the nofill/nostroke ones).