Prisms do not change the focus of a lens, but shift an image laterally, vertically or diagonally as required, so that if your eyes will not line up, the images will.
Suppose a right eye tended to look 5 degrees to the right of its partner. What the left eye saw as straight ahead, the right eye would see as 5 degrees to the left of straight ahead: double vision. (draw it on paper if it helps)
a prism shifting the image 5 degrees would let the right eye agree with the left as to what "looked " straight ahead, *even though the eyes were not straighter than before.
In practice the prisms do not usually have to be in front of the non-straight eye, as long as the total amount of prism is appropriate and, for appearance, often half the prism is placed in each eye, to produce the same total effect.
If you still have some double vision or eyestrain, even with best prisms, consider and consult as to whether a back-up pair of glasses with one side frosted or blacked out might not be restful for occasional use. Experiment with a folded piece of paper to cover one eye if you like...
