You cannot, because its technically impossible.
Routers are not smart enough to be able to identify when incoming traffic for a port has a special destiny on your LAN. So in order to know for the router where traffic needs to go, you forward incoming ports to a destination.
DMZ basically says: forward all ports to one IP. You can't just DMZ 2 ports and expect the router to copy the traffic to both routers.
The first device that would respond with an ok gets the connection and the traffic would never reach the other device because of it.
What you want to do is create a range of ports and dedicate that to one of the routers, and create a seperate range and dedicate that to the other router.
For example all ports from 10000-11000 are for router 1 and ports 12000 to 13000 are for router 2. DMZ is not used at all.
If this is not acceptable, put all networks on the first router and make it do the port forwarding as required.