I use a DX6i for a few different tanks as well as other stuff, so yes a multiple model Tx is a plus, especially if you can get inexpensive Rxs (tank Rxs don't need to have a quarter of a mile range).

Just keep in mind, you'll need both sticks self-centering.

All that said, the Futaba Attack is my absolute favorite for Tamiyas and given the choice, I'd probably pick an Attack for each over the Spektrum for multiples. Going with conversions, etc, I'd choose Spektrum for the ability to fine tune.

As of now, on Attacks, I have 3 Tamiyas (JS-2, Ersatz Panther, Pz IV). On the one DX6i, I run 3 Tamiya conversions (Initial Tiger, T-34 76, Su-100) and the Xion Kettenkrad.