Milkweed Longhorn Beetle (Tetraopes basalis, Cerambycidae), Yuba Pass, Sierra County, California - July 2008. Thanks to Ian Swift and Bug Guide for species-level identification. At one point, I had thought this was T. femoratus.
I think these two may have been battling. The milkweed is pretty finished and burnt in the mid-summer heat.
Several observed, also mating pairs. Seen nearby to Tetraopes melanurus http://www.inaturalist.org/observations/98177