You're in the situation I found myself in several years ago - Kids are getting too big to be towed and they're not yet ready for riding serious distance.
I didn't end up taking them on a GVBR when they outgrew the trailer. My wife and I had a little holiday by ourselves while the kids visited their Grandparents for the week.
Been a while since I towed my kids. Now they're big enough to ride themselves but they don't seem keen on a GVBR.
Things I considered:
Tag alongs - you seem to have a handle on these - There's a tandem Weehoo, but I'd think your boys are too big.
I tried a tandem Tag along, but the weight is prohibitive as the kids get bigger and the dual load moving around makes control challenging.
I looked at a Burley Piccolo and thought the design was a good idea.
You might be able to adapt a regular tag along from a seat tube to a rack mount.
There's a few options around from finding one with a really small stoker rear,to fitting crank shorteners or a kid back arrangement.
Lot more expensive though but far more controllable.
Tandems are quite niche and there's not a lot of LGS support around.
There's a Yahoo group that may offer some resource - there's always people moving equipment on and upgrading.https://groups.yahoo.com/neo/groups/sydneytandems/info
More esoteric options might be to look at a triplet or a quartet - Expensive and rare consigns that into the too hard basket.