Haven't actually tried this, but have done some similar stuff. Can you
set the gateway of the second router to the ISP gateway? Usually,
you'd set it to the first router's address -- then you'd see its
members.

What are the settings for the two networks? Unless they're different
nets (e.g. 192.168.1.x and 192.168.2.x), they'll definitely see each
other.

As you can see, I'm groping and could use a bit more info. Hope
someone else has actually done what you're trying.