What are the stem angles and length?

If you know all the angles and the sizes of the stacks you can use something like this website to see what your options are.

If the stems are the same length but different angles then I'm not sure how possible it is to replicate the exact same position. Spacers change where the stem reaches to on a line parallel to the steerer. Your different stems will reach to different parallel lines and since they won't intersect they can't be made the same.

Unless you are making up the difference by changing pad position for instance then the website will help you work that out but it may not be to the mm since pads tend to adjust by particular step distances.

Maybe one inverted does can be in the same position as the other or maybe I'm just wrong and haven't thought it through correctly. But the website should be useful. Give us the exact figures if you want to double check it.

Since stem lengths are usually in 10mm increments (and occasionally 5mm), there is no way to get bar placement to the mm with 2 stems of the same length and different angles. Arm cups and extensions are a different matter - depending on adjustability.

stem a is 120, 15 degrees, and has a height of 40mm.
stem b is 120, 10 degrees, and a height of 30mm.

If I assume you have a head angle of 72 degrees just because my bike does (but a small difference in head angle won't change things). Also assume you are angling both stems down and (b) is the new stem. If you removed 6mm of stack spacers then stem (b) will reach 3mm further than the other but the heights will be the same. It's actually more tricky than that in the tool because, as I read it, it uses 'height' as the height of the center of the stem. So a combination of spacer height and half the stem height. At least that's the way I read it so I thought I'd spell it out so someone could double check the workings.

TLDR - pads 3mm back, remove 6mm of stack.