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I've downloaded My Trails maps by jbensman and assumed I could use it as an overlay of the topo maps that where factory loaded onto my Montana and are in BaseCamp.  But I haven't even gotten to the handheld yet... just noticed in BaseCamp that I can only use one map or the other.  Not very useful this way so I assume I'm missing something. I've tried "shift" and "ctrl", "fn", "alt" but I cannot load up two maps sets at once. Can someone explain?  Is there a set of YouTube tutorials?