The first problem would be getting a derailleur hanger on the Langster. I've worked on some Peugeots which had semi-horizontal dropouts and a metal insert into the right dropout that acted as the hanger. Even if you could fabricate that, it's still a problem. Let's not mention rear wheel spacing. It just won't work.

I'd think your options are to rig the TT bike the best you can, or perhaps try something like the dinglecog on the langster.

You could even go freewheel and put two different sizes on it. That seems like a good compromise.

If you choose the Tri bike, you might want to find the biggest touring saddle bag you can and hope that will work.