Can 3 ports on a switch be configured to connect the WAN cable (from the modem) to either router A or router B?

[ErniePantuso]

I currently have the cable from the modem connected to one of two ethernet couplers - one of which is connected to the WAN interface on my pfSense router, the other to a different (old/consumer grade) router. The couplers are taped together and it's a simple matter to move the modem cable from one to the other - but if possible, I'd rather do this on the switch. TP-Link tells me I can't do this in the switch config - will pfSense let me do it? If I can do it, will it still work if pfSense/router goes offline? (The old consumer grade router obviously doesn't have the capability to do that kind of routing/port management.)