The I2C address of most of mindsensors' devices can be changed.
Download the address change program from here.
(This URL also has program to scan the attached sensors and report it's info on NXT screen.)
Please note down your address carefully for future reference, and use the new address in your programs (such as NXT-G block).
If you are connecting single device to NXT sensor port, you don't need to change address.
As per I2C specifications, you can connect many devices on a single I2C port. However each device must have unique I2C address.
So, you could connect two (or more) NXTMMX-v2 to a single sensor port of your NXT, but you will have to change one of the devices' address so that it differs from the other.