I still think this might be a power supply problem.
If you are running say G0 X10 Y10 from a position of 0,0 then as you say this is 45 degrees. Yours motors will be taking the maximum current exactly simultaneously. THEY ARE INSTANTANEOUS MACHINES AND REQUIRE MAXIMUM CURRENT IMMEDIATELY FOR A SHORT PULSE THEN STOP, then the start again, and stop etc etc.
I assume they are both set up for the same number of steps per rev or per inch, or millimeter, whatever, so the will perform simultaneously, and (of course) since they are controlled via a parrallel port it will be simultaneous. Even if you can only detect a 1 volt drop on a multimeter, your actual voltage drop will be greater.
Try your power supply running a large motor or something, and try stalling the spindle and see if the voltage drops when you get to say 10 amps. It must a a big **** power supply to give 7.5 to 10 amps without loosing voltage.
Jim