According to HorneTD :
rider89 wrote:
wanting to feed a branch box with a 60a breaker (#6 copper) thru EMT.

This may be a stupid question, but is there anything in the NEC to
prohibit using double #10's to get the amperage?
Reason is I have plenty of (free) #10 on hand and tube capacity is
slightly better with #10's.

310.4 Conductors in Parallel.
Aluminum, copper-clad aluminum, or copper conductors of size 1/0 AWG and
larger, comprising each phase, neutral, or grounded circuit conductor,
shall be permitted to be connected in parallel (electrically joined at
both ends to form a single conductor).

Does it not also say that _each_ conductor must be the same size, and
each capable of carrying the full breaker current?

[Our code does.]

Which basically means you can parallel conductors to reduce end-to-end
resistance (voltage drop), but NOT to increase circuit ampacity.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.