I would wonder whether you couldn't do something simple like 1.25 TT, 1.375-1.5 DT, 3/4" straight for the stays, all .035. I'm sure you can get by with 1.125 head. That is overbuilt for just about everything that it is put on, so for a comuter, no 30 foot drops... Probably fine.
You can throw in a chainstay that is tapered, though it isn't really as efficient as a straight tube in this application. But it makes it look like a bike. A tandem CS is OK, but some of them are like truck axles so be careful which one you order. A stiff touring weight stay is all you need, bumped up a notch.
There are some super heavy tube sets, but all they really do is hover a thou off the straight tube anyway, so you can suit yourself. I find it hard to believe that anyone really knows that a butted tube that is a radom thou thinner is actually working any better. But often the butted is cheaper and doesn't require as much cleaning, so it is easier to use.