Hello there,
Without "vlan-tagging" knob, only one unit is allowed on GigE interface and it has to be unit 0.
I think static IP Demux interface fits your requirement but it is supported on MX or M120 only
http://www.juniper.net/techpubs/en_US/junos10.1/information-products/topic-collections/config-guide-network-interfaces/topic-25484.html
As to other router models, you could do the following:
-- if you need more than 1 IP address to be present in the same LAN, then please configure secondary IPs on unit 0 family inet.
-- if you need 2 untagged Ethernet logical interfaces in the same LAN for the purposes of shaping/CoS etc then you could use 2 physical ports or alternatively look into "virtual-channels" feature on J-series routers.
HTH
Regards
Alex