KimberKable makes some nice in-wall cable (12 gauge) but be careful about the gauge requirements on the speakers and, if applicable, in-wall volume controls -- the latter are usually 14 gauge so require some extra work with the Kimbers. Audioquest also makes some good cable and is on sale if you look out for the banner here on Audiogon (I'll check to get details tomorrow). If it's for rear channels, those may both be overkill (in that case, Tweeter has some decent cable for less money -- four 16 gauge wires you can tie together to 13 gauge and it sounds very nice).
I've looked at a lot of in-wall speakers lately and have not seen bi-wired capacity so that may be a moot point. Shouldn't be a problem to tie the runs together to one speaker if you set up the cables for bi-wiring but can't find the bi-wireable speakers. In fact, thicker cable lowers resistance. But, be careful about how much gauge the speaker can handle. If you use two runs of 14 gauge per speaker, that's a combined 11 gauge wire (two runs of the same gauge result in that gauge minus 3). It may not fit without removing strands. Good luck.