Well, i believe, that (per how i uderstand what manual says), you get each sub individually adjusted in 2.2 configuration (if you follow cited recommendation).Sure, I know what the manual says. You can(obviously) use more than one subwoofer *in general*. But GLM doesn't do real multi-sub, by which I mean, setting delay, gain, and EQ filters separately and *differently* for each sub to achieve smooth bass overall. That is what MSO & Dirac DLBC do.
That doesn't make GLM bad, but it is a limitation. For one sub, it doesn't matter either.
I guess, that for multichannel, you must use different Subwoofer Group ID for each subwoofer to get the individual adjustments. But that is not described or explained in details.
PS. I am in a search for a subwoofer that will serve LFE channel in 5.1 setup.