Assuming you are using %volts mode, it is equivalent to driving it with a PWM Y-cable and is very doable. Simply send the same command to each jaguar on the same side of the tank (four commands instead of two). Theoretically, you would need to put them into a group and then send an additional execute group command, but we have not found that to be necessary.
If you are trying to use the PID feature of the jaguar with encoders hooked to the jaguar (as opposed to the cRIO DSC), then things get much more complicated. Teams have had trouble splitting the encoder to go to two jaguars and even if you accomplished the split, you have two servo loops which will fight each other for control of the encoder velocity. Multi-motor PID is much simpler with one controller (cRIO) and then send duplicate commands to the jaguars in %volts mode as above.