I am pretty sure you can only use a single relay on the Gmajor with the dual recs, TC Electronics site mentions something about the common ground being a problem on the rectifier series.
The Axess Electronics product can do the channel switching (there are other similar products as well) but I got such great service from Mario and love the product that I can't help but point it out.
I was not totally happy with switching channels on my Mark IV with the GMajor either.