If I'm understanding what you're trying to do : (Equalize the felt recoil in both your wife's SP101 .38 special, and your SP101 .357 ?) Yes, you can more or less do it. Assuming both guns have same barrel length and are about the same weight, and you are using same bullet and powder for both loads, then equal velocities should produce approximately similar felt recoil. So, since with equal amounts of powder in both the .38 and .357 cases, the .357 load will be slower (In this example, bigger case=lower pressure=lower velocity) you will need to slightly increase the charge in the .357 load until the velocity is equal to the .38 load.
If that's not what you're trying to do, please clarify a bit more, and we'll have another try at a solution.