Is it possible to use 2 rotary encoders at once?
It depends on how the first is configured. If it connects the output to ground and uses a pullup resistor to hold the output normally high (or vice versa) then it is possible to put a similar device in parallel.

The tricky part is finding a device that uses the same output encoding. Some are pulse per detent, some are two pulses per detent. Make sure that the new device matches the original device's encoding exactly.