The Encoder library will use 2, 1 or no interrupts. So you could use pin 2 and 4 (for instance) for 1 encoder and 3 and 7 (for instance) for the second encoder. That may be fast enough and still allow 2 encoders. If you don't need a lot of speed from the second encoder (human interface), use 2 non-interrupt pins and save the interrupts for the encoder that requires speed.
From the encoder library docs:
Encoders have 2 signals, which must be connected to 2 pins. There are three options.
- Best Performance: Both signals connect to interrupt pins.
- Good Performance: First signal connects to an interrupt pin, second to a non-interrupt pin.
- Low Performance: Both signals connect to non-interrupt pins