Separate packs.
For wireless guitar, I have a Sennheiser transmitter on each of my guitar straps, in a little neoprene pocket. I leave them both on, and just mute all of them except for the one I'm currently using. That way I can switch guitars in a matter of seconds without having to make any other changes.
For wireless IEM, I have a Sennheiser receiver that I clip to the back of my belt, so it's situated in the center of my low back. I've found that is the most comfortable place for it over longer shows, and I can easily reach it if I need to make an adjustment. I run my IEM cable up my back, under the top layer of clothing; that way it can easily slide between the layers of fabric. If I'm only wearing a single layer, I keep the cable on the outside.