In article , "toller" wrote:
My understanding is that #8nmc can be attached to the bottom of joists, but
#10nmc must run through holes in them. Is that correct?

Partially. 8/3 may be attached to the bottom of joists, but 8/2 may not.

I need to run two new cables, a 10/3 and a 12/2. If I ran them together,
they would have more mass than a 8/2. Can I attach them to the bottom of
the joists, or is code real specific on the matter?

No. Code is "real specific":

"Where the cable is run at angles with joists in unfinished basements, it
shall be permissible to secure cables not smaller than two No. 6 or three No.
8 conductors directly to the lower edges of the joists. Smaller cables shall
either be run through bored holes in joists or on running boards. Where run
parallel to the joists, cable of any size shall be secured to the sides of the
joists in accordance with Section 300-4(d).