The motors will run faster but have a shorter life if you raise the voltage. Since these are toy motors, we don't have any reliability or lifetime numbers even at 3V, so we can't say how much life you're losing with a higher voltage. In my experience, they haven't died right away at 6 V.
I think your second question is a series/parallel question. You should connect your motors in parallel, meaning that each one has an independent connection to your power source. Both motors should run as fast as just one motor; if you see the speed go down a lot for both motors, you probably have them hooked up the wrong way (in series). Provided your battery can supply the current, each motor will have the full battery voltage on it, not the battery voltage divided by number of motors.
If you want more speed, you might consider a different gear ratio, which you could get with the double gearbox, which we like more than the twin-motor gearbox.