For hauling things in the box alone, the bags are worth it. The trailer effects are a bonus.My trailer is 7k dry and 10 gvwr. I haven't fully loaded the trailer with the bikes and all the gear yet, so i don't know what the real number is, nor how having so much weight (streetglide + softail) in the back will affect it. I also put loads in the bed a few times a year, up to a pallet (ton) of wood pellets, so the bags would be nice for that too.
I'd say your two bikes are within the ballpark od my ATV, maybe a little heavier.

Harleys are some heavy assed bitches.