I need to install baseboard electric heating units in two rooms I'm
refurbishing. One will need a 48" 2000 watt unit; the other a 36" 1500
watt unit.

The instructions in each box say that I will need to use a 220v
circuit with "amperage according to local code".

Both rooms are next to each other and I'm wondering if instead of
fishing two wires, I can go with one 10/2 30amp circuit and have both
units branching off the main line. What might be the minimum wiring
and amp circuit? What would be safest? What's the most amperage I can
get out of a 10/2 line?

Related question: I've got tons of 12/3 wiring with ground laying
around unused. Can I turn this into, say, 10/2 by simply clamping
the black and red wires together at the panel and at the end point and
then painting the red wire with black marker pen to indicate power? I
hate to waste wire with the cost of copper these days.