Do two encoders connected to a microcontroller interfere with each other?
Don't ever tie external inputs like this directly to interrupt pins. Interrupt pins should be used with "safe" inputs which can't run away and leave the controller a gibbering puddle of ooze as it valiantly tries to service endless interrupts.

(you won't actually kill the MCU, but your embedded application will simply not work)

For things like encoders it's best to use counter hardware and then have a periodic interrupt to look at the counts and determine how many counts per interval of time you're seeing, which will then tell you how fast the motor is turning. If you need directional information as well you could run the I/Q encoder outputs to the clock/direction inputs of standard up/down counter ICs and then poll them with the microcontroller.

I know this is probably not the kind of answer you were hoping for, but using interrupts to count encoder pulses is bad design.