Yes, a second moisture trap will slightly reduce pressure. Anything that is added in the path will cause a reduction in pressure. That reduction may be slight, even insignificant, but it is there.
Every airbrush is different, every pressure gauge is different, every installation is different. Just because Person Y is able to spray at 10 psi doesn't always mean you will be. Some airbrushes just will not properly atomize the paint at pressures that low while others work quite well. Also keep in mind that pressure gauges are notoriously inaccurate, especially at low pressures. I have two on my air line and at low pressure one will read several psi lower than the other. Which is right and which is wrong? Don't know, don't care. I just keep playing around until I find something that works.
A couple of words of advice:
1) If you are thinning paint 90:10 then all you are spraying is tinted thinner. If you have to go much past 50/50 something is not working right.
2) Acrylics dry notoriously fast, and you are probably experiencing some drying at the airbrush tip that is causing a lot of the splatter. Try adding some acrylic retarder to your paint cup to slow down the drying time and that should help.
3) If your airbrush doesn't work right at 7-10 psi don't worry about it. Find smoething that works for you and stick with it.