Will this setup work?
Can I eliminate either switch?
And Use 1 router to pass the 2nd Public IP to 2nd router?
Will it work with 1 AP interfacing to 2 routers each with own hotspot?
Can this be done with one router?
Thanks

Depends. You can do more than one Hotspot per router, yes, so you could do just one router. No different from running just one. If the ISP uses static IPs just assign them both to the outside interface and instead of masquerade use src-nat with the appropriate to-address to NAT the two Hotspot networks to different IPs, if you need to. Otherwise just use one IP for both. You can use just one switch (even if you keep two routers) by getting something that can do VLANs (802.1q). There's cheap dot1q gear around. Each Hotspot network and potentially each WAN uplink if you use two routers gets its own VLAN.

Maybe get yourself a CCNA book or something. Ignore the Cisco specific parts and study just the background material. While not completely trivial these are fairly basic things to understand.

I'm primarily concerned with making things work on the budget I'm on.
I have an existing userbase of customers running through the RB750, with level 4 lic.
Presently there's @ 10 -20 active users at a time authenticating with UserManager
I'm plannng for an upcoming event which will have up to 50-100 active users with will need to be using PayPal/Usermanager.
So I've added the second box with routerOS 4.11 license level 5.
I need to make sure I don't max out that - usermanager with the allowed 50 active sessions.
I'd like to avoid using Hotspot database as much as possible and utilize UserManager/Paypal.
The solution would be to buy the $250 license, but just can't do it right now.
That was really what pre-occupied me instead of thinking through the aforementioned config.
So, that said. I need to use the box and the routerboard to get as many users in the Usermanager scenario.

Budget constraints aren't fun. Still I think VLANs are the way to go. You can pick up a used Dell PowerConnect for $20 and use just one switch for everything, there are other options. Hell, the 250GS from Mikrotik might (should) work for this. As long as the APs can do VLANs and support multiple SSIDs you can do both networks on each AP.
But if you really need to balance things, do just one network per AP and keep an eye on things, and move the APs over to the other network as one network has too many users. Try to plan out the area and where people will be so you can minimize that. It's absolutely doable, though.
Given your constraints of course you cannot eliminate either router.

Budget constraints aren't fun. Still I think VLANs are the way to go. You can pick up a used Dell PowerConnect for $20 and use just one switch for everything, there are other options. Hell, the 250GS from Mikrotik might (should) work for this. As long as the APs can do VLANs and support multiple SSIDs you can do both networks on each AP.
But if you really need to balance things, do just one network per AP and keep an eye on things, and move the APs over to the other network as one network has too many users. Try to plan out the area and where people will be so you can minimize that. It's absolutely doable, though.
Given your constraints of course you cannot eliminate either router.

You really ease the burden fewi.
Thanks again!.

My radios are UBNT.
From what I undersand UBNT is just now coming out, or will be, with firmware that supports VLAN.
Although, I believe I read someone has been making VLAN's work on the present firmware.
Just to clarify.
I won't need 2 or more SSID's per AP.
The AP's are physically @1 mile apart.
So at the terminal end each wireless AP would be given their own respective SSID.
So, one SSID will work off of the routerboard lic. 4. The other SSID would work off of the PC box with Lic. 5.
But, now I see the benefits of what you were suggesting with the vlan's and doing multiple SSID's per AP. And being able to use just one routerOS.