Compression....
To do a compression test, on one engine, take out both plugs, put the plug wires on the grounding lug, that, if your inside the boat looking back, should be on the right hand side. Then, thread the gage into the plug hole (there are adapters that you need to look at and choose the size that best compares to your spark plug). After you thread it in, turn the motor over several times. Then, look at the pressure. There is a little tit on the side that will allow you to bleed that pressure off. Do it again. Do it about 3 times to give you an average of the pressure. Then do the same procedure to all the plugs.
The ideal pressure is between 130 and 150. If you have 135 on the back cylinder and 145 on the front, that's o.k, but you don't want to have much more variance than ten pounds. Ideally, they'll both be within a few pounds of each other.
If the other motor has 120 and 125, that's fine too. The comparison between the tow motors isn't necessary. It may tell you which motor has the most wear and need to be watched. But as long as both motors are over 100 to 110 pounds....you should get a few more years out of them, if you take care of them.
The Challenger 1800, with the twin 787's is a really nice and powerful boat. You should be able to put at least 4 more people in, not counting you, and still go out and have fun. With some people, their combined weight may be so much that you can only put in 3. What I mean by that is total weight and how that weight is displaced in the boat is what you have to kinda figure out for yourself. But under normal conditions with normal size adults, you should be able to fill each seat with a person and still have fun. Now, by yourself, you'll see what kind of power it generates and how the performance is greatly affected by the amount of people you put in.
Good luck