The SRX only supports 1 loop back interface but you can set multiple IP addresses and then use the local-address command to reference it.
Please be aware if this is in a cluster and you want to terminate VPN'S On the loop back like you would On an ASA it is not supported but will work on a standalone box.
root@SRX110-1# show interfaces lo0
unit 0 {
family inet {
address 192.168.1.254;
Address 192.168.2.254;
}
And then if you were using this for terminating VPN's you would set the loop back as the interface and then add
Local-address 192.168.1.254----or whichever addresses you had configured you wanted to use
Hope this helps
SilentBob