is it safe to wire two runs of 12/3 romex in parallel for a 30 amp
subpanel in my shop? Ideally, I would use 10/3, but I have lots of
extra 12/3, and nothing to use it on. Thanks! Also, would it be up to
NEC code (in California if it matters)?

Safe/ sure,as long as the wire fits into the connectors.I'm not
familar with the cal codes but they surely will allow this set up.As
an added reason for them to approve this unstandard style try wraping
both runs together.Black tape will do wonders here.

What you described would actually be in violation of the NEC (National
Electric Code). The NEC allows only conductors of size 1/0 AWG and
larger to be wired in parallel (with a few very limited exceptions
that you would not fall under). #12 wire would be too small to be
legally run in parallel. The relevant code reference is NEC article
310.4.

The NEC code would apply in California. If a building inspector is
reviewing this installation, they would likely make you use the #10
wire.