It is possible, however, I would like to ask how far physically separated are the devices, as each RUT will have to be configured to act as a secondary wireless WAN source to the other.
To proceed you need to configure both RUTs on different LAN subnets, for example one having 192.168.1.0/24, the other 192.168.10.0/24. This can be set in Network -> Interfaces -> General -> [lan settings].
Next, configure each RUT as a wireless client to the other. Follow instructions in this video.
Lastly, enable failover for mobile and wireless interfaces in Network -> Failover page on both devices.
Now each RUT should treat the other as its failover source.
Alternatively, both devices could have LAN to WAN port connection, but due to RUT240/241 having two ethernet ports only, additional switches would have to be introduced.