Photoshop kinda demands a lot. Then again, it has multiple uses and kinda depends on what version you're running. While I do use Photoshop, I hardly use it in my art. Very rare exceptions have I used it between it and SAI.

....That gives me an idea...Would it be possible to use both programs for one project?

Technically, yes. SAI can open, edit, and save .PSD files, however the outcome of the image may vary. This may have to do with the fact that Photoshop is still an upgradable program, while SAI (in it's current English release) is still rocking and stable from the past. But hey, it if ain't broken, don't fix it.

Also note that Photoshop as a larger array of layer types and effects, while SAI is very limited, so trying to open them back and forth should be a warning right there. When in doubt, make backups of WIPs before attempting to use both programs for one pic.