To run dual drivers you just need to hook them up in parallel and add up their combined current. I will test them individually then with them wired together. You may loose 1-2mA when reading their combined output. It has been said that it does not matter but I like to set the drivers as close to the same mA as possible.
When I say parallel I mean both wire the corresponding wires from the driver together. IE both positive output wire from the driver will go to the positive diode pin and so on for the rest. Now you can heatsink the drivers or not. If you are running say 900mA each they can go for about 5-10 minutes before overheating. But as the driver heats up there will be a slight drop in power and if you sink them you can run them continuously and without this power drop. Shoot me a PM if you have any other questions.
Morgan also did a nice thread on his dual microboost build but the flexdrive is easier to sink and is a little more efficient with current when boosting the voltage. Hope this helps.:beer: