I wouldn't have thought so with my wife, but really, two gallons of water is plenty for two to share (get wet, soap up, wash the soap off). Not to mention, just with conserving water. When we're out camping, we figure we need four gallons a day (two for drinking/cooking, two for a shower).
I have a two gallon solar bag which I would actually just boil say four cups of the water and it would get the water temperture prefect when I didn't want to wait for the sun to heat the water or it was later in the day. But even being just two gallons, it's kind of heavy to hang the bag, and with a five gallon bag, that would really be heavy. I most recently bought a Nemo pressure shower, which I haven't used yet, but elimnates having to hang a heavy bag. Hope that helps, as with my wife, she's happier when she gets a shower each day.