It's interesting that you found these two quite similar looking species in the same localized area, and that they segregate according to host plant. Reinforces the notion that they are truly reproductively isolated groupings...from the image it's clear they maintain their distinctness of characters.

Also reinforces the notion that the previous association given in the literature for Artemisia californica as a host plant for T. luteocincta may be in error, and that Isocoma ssp. and close relatives may be the sole host plants for luteocincta.

This photo does a beautiful job illustrating subtle contrast of diagnostic characters for luteocincta vs. sericotrachyla, respectively:

        green vs. picious pronotal spots;
        shiny vs. dull pronotal surfaces;
        bright vs. subdued green of elytra;
        larger vs. smaller overall size.

...and it also gives an endearing display of inter-species camaraderie :-)

A further idea...inspired by your nice photo below:

 

How about a "pair photo" like the one here...but showing the ventral sides, and thus boldly illustrating the contrast between the entirely dark vs. medially pale-yellowish venters of luteocincta vs. sericotrachyla?

It would be a challenge I know...might need to temporarily place them in a cooler to immobilize them first, in order to pull off such a shot.

Moved from Trirhabda.