your 2950 is only layer 2, it will not do L3 that I am aware of..
so you have 250 and 251/24 running on the same layer 2 or do you have this setup with vlans using pfsense to route these?
Why don't you just connect your buildings with a transit network between your pfsense and then you could just use policy based routing for any client in building A to use the internet in B, or you could have B use internet A if you wanted, etc..
Since you show a client on that 251 segment this is clearly not a transit network.
If you connected your building correctly, simple routing/firewall rules to allow whatever you want to use whatever wan connection in either location. You could have multiple networks in each location, etc.
Done correctly you would never have to change a clients gateway, done correctly you could even leverage the wan in each location for load balancing, nor would you have to do any natting between your rfc1918 address space, etc. etc.
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