...But I can't hear anything on the output...
And you won't if, as AA4PB suggests, the radios are too close together.
That's because the transmitting radio overloads the receiving radio. It can't hear the
transmitting radio because it is on a different frequency, but the strong RF field typically
causes the receiver to become "deaf" to other signals (including the much weaker signal
back from the repeater.)
I notice this when operating mobile if I get within 2 or 3 car lengths of a station that I'm
walking with (usually because we came to a red light - on the open road with longer
spacing between us we can talk through the repeater just fine.) You might try putting the
second radio across the yard and turning the volume up full - that might be far enough
away to avoid the RF saturation. Or not...