Replace both seals regardless of results of pressure test? Or change both seals and then pressure test to confirm seals are set?
It is advantageous to know if the seals are leaking. The pressure test and soapy water should tell you.
When you ask about BOTH seals, are you referring to the fact that there may be two seals on a shaft or are you referring to the shift seals and drive shaft seals?
Regardless, if either has two seals, one on top of the other, you should replace both. It is often not possible to remove one without damaging the second.
There was a point made about springs on the seals. A lot of the seals have a groove that has a circular spring in it to keep the seal snug to the shaft. Those springs will often break into pieces. It you see that, you need to replace seals even if they are not currently leaking.
Note that when there are two seals that may be installed in opposite directions. The seals 'seal' in one direction of fluid flow. One seal stops oil from leaking out and its partner stops water from seeping in. That is why a true test of a lower unit includes a pressure and a vacuum test. Since you are leaking oil, the pressure test will show the leak.