Configure 1524 switch with 2 ISP connections

Sorry if this has been posted somewhere already, I haven't found the solution I need. 

I am configuring a 1524 2-switch stack with redundant ISP connections. This is my first go at something like this. Basically I am trying to take the Primary ISP connection and plugging that into port 1 of both units. Then on port 23, I want to connect the secondary ISP, again, on both units. My question is well, how? 

The client is expecting us to connect the switch directly to the ISP, however, there is a Watchguard active/passive firewall on site. I think I should have a connection for each ISP on the firewall then a link to each switch. That way, the firewall would be managing the failover. BUT is there a way to configure the stack in the manner that the client is asking?

thanks for your help