You can't route IPsec, so it's really a question of IPsec Phase 2 settings.
You would either need two separate tunnels, one for each subnet present on the side with the DMZ network, or they would have to be close enough in numbering that you could just specify a subnet mask that would cover them both (but not the network at the other site).
Or just ditch the IPsec tunnel, put in OpenVPN site-to-site shared key, and route however you like without the headache of IPsec. :-)
It's easier on 2.0 though with IPsec, you can just specify multiple networks under a single tunnel.