As Tabetha said could be a stuck relay or possibly a faulty fan switch.

When the faults happening again, unplug the fan switch. If the fans cut out it will be a switch fault. If they stay on, its likely to be a relay issue.

As for only one working, when the fans are in check to see if you have voltage to the fan that isnt working and also check for a good earth.

If both elements present then it may be a fault with the fan itself. if not you've got a wiring/power/earth supply fault.

Personally I would just pop a new fan switch in and rewire the fan circuit using 1 relay and 1 fuse for each fan. A lot more reliable!!!