To answer some of the unknowns referenced in your response:
"But is it not clear whether this is different from what you do with the old ISP or is the same."
Currently, all outbound and inbound traffic traverse ISP1, since it is the only ISP in our environment. The idea is to design a way to have both ISP's connected to the router. 1. Allow outbound traffic to use ISP2 (currently the default route is set to ISP1). 2. Allow a transition to move Internet hosts located in the DMZ area of the firewall to ISP2 (we are implementing a new domain name for this connection that allows us to move piece by piece versus all at once).
"There is an aspect of this design which you have not mentioned and which will impact your options for using both providers. I assume that network address translation is being done. Is it done on the firewall or is it done on the router?"
All NATs are being done on the firewall currently. I'd like to keep this function on the firewall and just allow the router to determine which ISP traffic is sent outbound. But if needed, I can move the NAT to the router. On the firewall I will create a second NAT group for the new ISP. The firewall will have the same default gateway (the router).
I appreciate your response.