Define WAN2 using private IP

I currently have 2 DSL WAN on Balance 20 (same physical location). Working fine except low bandwidth.
I have fiber run to separate building to connect LAN segments (all LAN IP in
I want to add a cable internet connection in the building where there are NOT DSL connections and drop one of the DSL connections. (take Balance 20 out)
I have a Balance 310 available and was thinking I could configure WAN1 for the cable internet connection and then configure WAN2 with GW (DSL modem).
Here is an image. (not current config, proposed config)

Will this work o.k.?
If fiber goes down, health check to (on WAN2) will fail and all traffic in/out WAN1.
If cable internet health check fails (WAN1), all traffic in/out WAN2 (ethernet over fiber to segment that has DSL modem).

Is there only a single fiber link? If that is true, how do you plan to separate the LAN users from the network for the DSL modem?

That is my current issue. It is currently a transmit/recieve pair of fibers using media converters…so appears to be transparent.
I have tested so far by adding IP in on 2 PCs…one on each side of the fiber, and they ping. (although I haven’t done any testing other than that yet)
I do actually have more fibers and might even have a spare set of media converters. I’ll have to see if I can get a separate line up for just, but was hoping to just get away with running 2 network IPs over the same physical cabling.
Is that a big no-no, or is it doable?

I would recommend using a separate transmit/receive pair if possible, or alternatively you could separate these two networks with VLAN capable switches if the same physical cabling must be used.

Thanks. I forgot that I actually have a pair of media converters on cold standby by (connected to fiber, not powered up). These were in case active set goes down. Currently fiber is only thing tying two buildings.
I will have to make 50 foot run of Cat6 from DSL box to fiber converter and dedicate the 2nd fiber link to…but sounds like it is the proper way to do it.