Just get the 70D. It is less expensive than another M10 and you won't have the issue of having to sync two separate recordings in post.
Mics: akg c460 (ck61, ck63), c414buls, c568eb; at4049a, 4051a, 4053a, at853; josephson c42; neumann U87, km84i; beyer m130, m160, m500; aea r84; gefell m71, mt711s, m200, m201; sony c38; schoeps cmc6, CMBI (mk4, mk21, mk41, mk4v); sennheiser mkh40, md421, md431, md541
Pres: API, a-designs, pendulum, purple, millennia, gt, littlebox, tinybox, usbpre2, CA 9200, pipsqueak, pueblo pending
Cables: KCY, CMR, Naiant AKG actives, PFAs, asst. GAKables and Darktrain
Recorders/converters: dr680, m10, dr-2d, d50, dr70d, zoom f8, lynx aurora 8
Yeah, I'm an attorney, but everyone needs a day job!