SRX5308 and DHCP relay

First - I'm not certain I understand your question, and second - I don't have an SRX5308, but it should be similar to the way my FVS318N works. I'm going to assume that your concern is centered on how the DHCP server knows which DHCP scope to use on a given request - this is not really a Netgear issue, it's more of a "how DHCP works" type deal. I assume you're running VLANs and you mean VLAN1 & VLAN2 rather than LAN1 & LAN2 - just set the appropriate VLAN addresses on the VLANs and then enable DHCP relay in the DHCP setup for VLAN2 and point it to the address of the DHCP server. The Windows DHCP server can tell where the DHCPDiscover request comes from - if it is received directly on the server's NIC at 192.168.4.2, it will make a DHCPOffer from the 192.168.4.0/24 pool, however if the DHCPDiscover request is relayed from 192.168.5.1, it will respond with an offer from the 192.168.5.0/24 pool. It's kind of a puzzling thing when you first think about it, but it actually is very simple to configure - try it and see.