Yes, it is possible for both to have the same address. Just configure fxp0.0 in the main section of the config with a single address, commit synchronise and you're done. However, this has some significant issues. First of all, you will probably see duplicate IP addresses on your network if both are physically connected!
It is possible for both REs to share an address that is only active on the master RE. You can configure this under groups as follows.