Dual ISP (1-StaticIP-Bridged-SlowUp, 1-DHCP-FastUp), Single OWrt Advice

Can you have internet traffic inbound to public static IP's, come in on one ISP/gateway that provides the public statics, but have the same inbound traffic reply/go out a different ISP that has faster upload speeds?

I am essentially trying to use our low-cost DSL public-static bridged-modem slow upload speed ISP service, combined with low-cost consumer class DHCP high-speed cable ISP service, to avoid the very high-cost 3yr contract of business class public static bridged service that would be 300% more expensive in my area.

Network(s) support test lab of 32 networked micro-servers used by remote software engineers and QA for software testing.

I have two home ISP's. ISP1 is slow (1Mbs-Up) bridged-modem DSL with static IP's, ISP2 is consumer class cable DHCP fast up speeds.

Any suggestions on some kind of dual/multi-wan OpenWRT configuration that can support inbound public static traffic that goes back out a different ISP/gateway?

If not, then any suggestions for dual-WAN configuration for dual-ISP's?

Should both ISP's route/NAT into the same/combined network/subnet?

Right now the two ISP's are two separate internal/external networks with static routing between the two internal networks.