Well, I am not fully understanding what you are trying to do other than share an Internet connection with two distinct businesses.
... and access both networks from one computer... Ideally each business will be private from one another; this simply makes sense.
Here is how I would consider setting it up with junk home or soho equipment (technically there will be four(4) subnets counting the Internet as a subnet):
0. Internet (to be shared; this is your first subnet you don't even configure it the ISP does), so plug it into the INTERNET or WAN port of a simple WIRED router... The LAN ports on this router should have DHCP turned on to configure the INTERNET or WAN ports on other routers. Let's say this router hosts 192.168.1.0/24 on its LAN side... to other routers, it will look more or less the same as your modem. It WILL do the Layer-3 translations between 192.168.1.0/24 to the real CIDR block provided by the ISP... Simply treat this as if it multiple private modems. I would probably turn on ISOLATION, too.
1 Connect another router... this time a WIRELESS router... for business #1. Plug its WAN or INTERNET port into a LAN port on the aforementioned router in #0. It's WAN port should get an IP address on the 192.168.1.0/24 network likely with a gateway of 192.168.1.1. Make certain to configure this router to provide non-overlapping LAN IP addresses. For example you might set it to provide 192.168.2.0/24 on the LAN ports. Now plugin (or connect wirelessly) all the business computers for Business #1... they should ALL have Internet.
2. Do the exact same thing with another WIRELESS router with its WAN or INTERNET port up linked to the WIRED router from #0. It's INTERNET or WAN port should be automatically configured with a different IP address from the router in #2, yet it should pull the same default gateway most likely 192.168.1.1 for this port. Again, make certain you do not have overlapping networks that is to say the LAN ports must NOT be on the 192.168.1.0/24 network. While technically you could completely reuse 192.168.2.0/24 since this is another private LAN, I would not do that if you want to be connected to both business networks at the same time, so use something like 192.168.3.0/24 for the LAN ports here.
Now, you have got two (2) business networks that cannot see each other, yet they use the same Internet connection. There is nothing keeping you from connecting a computer with multiple network adapters to both the Wireless/Wired routers described in #1 for Business #1 and Wireless/Wired routers described in #2 for Business #2.
The computer connected to both networks will be smart enough to know that if it wants to communicate with Business #1 to do it via the adapter connected 192.168.3.0/24 network and to connect to Business #2 to do it via the adapter connected to the 192.168.3.0/24 network. Simply put each business network will simply be a directly-connected, layer-2 network as far as the computer is concerned. For reaching out to the Internet it really won't matter if it uses 192.168.2.1 or 192.168.3.1; since, either default gateway on either adapter will be translated to 192.168.1.1 on the 192.168.1.0/24 network and then translated again to the ISP's network.
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Simply put... it should do what you want it to do; I think.