I see two big hurdles with your proposed "linking" of two. First is control. Worst case, you have to control each with its own dedicated surface. That's not a huge deal. Tablets are cheap. The biggest issue is the fact that you'll now have, potentially, several monitor mixes available, but the ones driven by the first mixer can only have that mixer's inputs in them, while the ones driven by the second can have the second mixer's channels, plus the combined "submix" input. Let me clarify. For the sake of this example, we're going to use mixer A for all the instrument inputs. Drums, bass, gtrs, Keys, horns, what have you. So, we mix all that down to "band, stereo", into mixer B. This one handles all the vocals for the music portion of the worship service, as well as the spoken word mic inputs. Great right? Yep. Now, the drummer's monitor mix needs him in it, as well as bass and keys. No electric gtr(he's too loud on stage), but a fair amount of acoustic, AND he'd like to hear the lead vocal stuff, the choir, and all the spoken word. That's gonna be your issue. If you drive all the monitors from the second, "master" mixer, you can't control individual channels from the slave mixer in the monitors. That reason alone MIGHT be enough to justify waiting for a single mixer that will do it all.