Technically, you can worship more than one of the Gray gods. I mean, you can worship however you like, but the gods don’t always acknowledge said method of worship. People pick all kinds of bizarre options: worshiping all twelve remaining gods, worshiping only the Three, worshiping the Destroyer, and disbelieving that the gods exist at all.
So you can worship two or more of the Gray gods if you want, but then you aren’t actually part of the Gray. The Gray gods are jealous. They’re willing to work with one another, generally agree on important stuff, and even work together when they have to. But they do not like to share their toys. The Light gods can share their worshipers, but the Gray refuses.
People who choose to worship two or more of the Gray gods get called Gray generalists, and proper Gray paths like to spit on them. These people are allowed to live on Gray worlds and visit all the various temples where they want to pray. About half of Gray generalists live on Gray worlds, while the other half live on Neutral worlds. Gray generalists cannot serve in the Gray military or vote for local representation on Gray worlds. They are second-class citizens. So, even though Neutral worlds aren’t as nice or as rich as Gray worlds, half of Gray generalists live on Neutral worlds anyway, because they get respect there.
There’s no such thing as a Light generalist, since the Light gods like each other. There are Dark generalists as well, who must be crazy, because who wants to be Dark but not have the protection, however minimal, of a god? Someone who worships more than one alignment, like someone who worships one Light god and one Gray god, would just be considered Neutral, since Neutral is synonymous with miscellaneous.