control signals to draw a circle

[stirling]

You're entering the field of "XY Interpolation". I wouldn't worry at this stage about whether the XY motors step individually or at the same time - that's way down the road and when you understand how and when to step you'll find that sometimes the X leads and sometimes the Y and sometimes they'll step together. e.g. You've given an example of a 45 degree line and in that "special" case they can/will/should/may (depending on your implementation) both step together (IF you have the same "steps per" on X and Y).

With regards circles - for example if you start at 9 o'clock and draw clockwise, Y will lead until somewhere between 10 and 11 when both Y and Y will step together for (some) period and then X will lead until somewhere between 1 and 2 etc. etc. i.e. you'll discover you're interested in "octants" (as in half quadrants).