All the power comes in from the single phase line.
SOME of the power passes thru the converter to the third wire output (manufactured phase). About 33% of the total power out is passed thru the converter,
SOME of teh power is lost in converter losses.
Current-wise, you have the draw for each motor connected, plus the draw for the converter motor, with the latter based on about 50% power maximum.
FLA for the 2 HP is about 7A, so 14 for the two.
A added current comes through the converter, another 14 plus losses.
The converter draws around 10 or 12+ amps on idle
You will have something around 40+ amps draw.
75C cable (use that rating based on allowed terminal temps) is 45 A, so you are squeaking by at full draw.
You are supposed to protect at 125% of max nameplate, requiring over 45A conductors so it looks like you should go to 6 ga.
But if you wire the phase converter with its own protection, and bring its output (manufactured phase only) over and combine with direct wired single phase, you do not have to have the phase converter input current included with the load motor circuit.
That is a recommended wiring method, and is fairly common. You can find details at the phase converter maker's sited... check Arco and Phase-A-matic
You would be fine with the 8 then.