According to the Star Alliance elite benefits guide for *G, "Priority Airport Check-in - allows you to check-in at the Business and/or First Class counter." (Bolding mine.) There is no official provision for guests.

That said, I've checked in with one guest travelling with me on the same itinerary many times, without the slightest hassle ever. I doubt you'd have a problem with two. However, it would be up to the check-in agent to permit it as a favo(u)r, as it's not an official entitlement.

Also, there are probably over fifty different versions of the *A elite benefits guide, when you consider multiple airlines, print vs. online, for internal vs. external use, in English vs. other languages ... You may find a version that's more generous than the one I used (from www.staralliance.com). If you do, you may want to print it out, or photocopy it if it's in hard copy, and carry it with you. You might also want to post it here for the rest of us!