Yes. You can have VLANs on WAN going to an outside switch to two or more different providers.
pfSense: vlan 100 vlan 101
Switch: tagged port vlans 100 and 101 to pfsense, untagged 100 to ISP 1 modem, untagged 101 port to ISP 2 modem.
It will be functionally equivalent to having two different WAN interfaces.
You will have to understand that powering off and on one of the modems won't trigger a DHCP renewal event on pfSense because the port will not go down from its perspective. You might have to release renew manually, etc, if that ever arises.