If both your WAN and LAN ports are Ethernet, then the difference between them is purely in software. Ethernet ports are always bidirectional – it's not like (e.g.) video, where some ports only have a transmitter and some only have a receiver.
In other words, every Ethernet port – whether it's labelled "WAN" or "LAN" – is always input and output.
The differences between them are mostly just IP configuration and stuff. While you could kind of say that the LAN port (with DHCP service) is an "output" and the WAN port (with DHCP client) is an "input", that's only kind of true – it's only correct for the standard configuration that home gateways come with, but it's not something that generally makes sense for router ports at all.
So if you have two home routers, and connect both of their WAN ports together, then things will not work at first (the ports will light up and nothing else) – but depending on the router's flexibility can be made to work easily. Specifically:
Both routers have a DHCP client running on the WAN port, expecting to obtain an IP address from a DHCP server connected to that port.
If the network doesn't have a DHCP server (i.e. it consists entirely just of the two DHCP clients, i.e. two "WAN ports", connected to each other) then none of them will be able to obtain an IP address via DHCP.
But if you set both ports to use static IP configuration, then they'll be able to talk to each other just fine. (The only thing that DHCP does is IP configuration; it doesn't actually enforce which way packets go.)
For example, if router A uses 192.168.1.0/24 for its LAN, and router B uses 192.168.2.0/24, then configure both of their WAN ports to use a static IP address 192.168.7.x/24 or something else.
Alternatively, the router might even allow you to enable a DHCP server on the WAN port (just like on the LAN side). If one router is the DHCP server, another can remain as a DHCP client.
Both routers come with a firewall which allows new connections in the LAN→WAN direction, but not the other way around.
This made sense when the "WAN" side was facing the Internet, but in your case you'll need to disable it. Many routers allow the firewall to be either disabled completely, or a new rule added that allows everything. (Not all do; in some models it's deliberately made permanently active.)
Both routers come with NAT enabled, rewriting the source IP address of "outgoing" (LAN→WAN) connections with their own address.
NAT actually shouldn't prevent connections, but it can cause some confusion – and again, it no longer makes any sense to use it for this kind of direct connection when ISPs are not involved.
This is sometimes disabled together with disabling the firewall; sometimes through a separate checkbox. Often it cannot be disabled at all.
Whether this has a "practical use", it depends. In general, yes; although in most situations you would want to be in 3 or more networks, and wouldn't be converting a WAN port to do something else, but instead would configure the router to have a 3rd network (say "LAN2" or "Neighbour's LAN") while the WAN still went to an ISP.
