I've been brewing 10 gallon batches, making 3 liter yeast starters with 1 1/2 cups dme (wyeast smack packs). I've done some of the yeast calculators that say that I should be using 2 packs of yeast, but I only use one. All the beer I've made have been fairly low gravity though.
I have noticed similar conditions in the past, and the result is a beer that has no head retention. I don't know why it happens or what is going on, but bio-foam helps a lot.
There is no advantage to this. The 5 gallons with yeast in it will do well, but the 5 gallons will be sitting there waiting for any bad critters to get going. Active yeast is the best thing your wert has going for it.