Two Internet Connections - One Network - How To Manage

This is fortunately not that hard to solve. Let's say we define the following: ISP_A and ISP_B being your two ISPs and CLIENT_NET_A (192.168.1.0/24) and CLIENT_NET_B (192.168.2.0) being the two different subnets in your office. Then what you could do to achieve what you want is this:

• Set up a router that has an address on both CLIENT_NET_A and CLIENT_NET_B, say 192.168.0.1 and 192.168.1.1.
• The router should also have connections to ISP_A and ISP_B. Not every router out there will do this nicely, but Mikrotik and many others will work for this kind of setup
• Setup NAT on the router so traffic going coming from CLIENT_NET_A goes to ISP_A and correspondingly for CLIENT_NET_B
• Still on the router, add a route from CLIENT_NET_A to CLIENT_NET_B if you want clients on the two subnets to able to communicate with each other
• Assign static IP addresses to the clients you want to be part of CLIENT_NET_A and CLIENT_NET_B, making sure to assign default gateway matching the correct subnet

You should probably also add a DHCP server to your setup so you can assign addresses to guests and you might even want to assign IPs to your clients via DHCP too - you would have to register each client's MAC address in the DHCP though.

With respects to the firewall, you might want to add a drawing to improve understanding of your setup.

That should cover it from a very top down view. If you need help with the specifics, you can easily find it online. While not entirely standard, your setup is not that different from a lot of other stuff out there.