you can introduce one more switch in between your two router and existing switch.
connectivity will be like this:
Routers(R1 and R2) --> switch--> steelhead-->switch-->lan users
steelhead will detect traffic on its wan and lan port with different mac address ,so it will not create any problem.
Guillaume is right but Ramesh proposed a workaround that I tested many times.
If you don't want this workaround that create a SPOF, the "official" solution (i.e. from deploy guide) is the Virtual Inpath setup. With Cisco routers, you can use WCCP config on both routers to redirect part or all traffic (ACL related) to the wan0 CX255 port on the LAN side of the routers (lan0 is not connected then). Please check the guide.
I tested this too with great success but :
1. Beware of MTU Path Discovery as WCCP involves GRE tunneling,
2. You'll have an extra complexity with Cisco WCCP & ACL config to manage (if ever you can do it)
Hope this helps.