How to do Bidirectional multiplex switch signal over two wires?
Have a look at DTFM (dual-tone, multi-frequency) chips. These are used in telephones. You'd need a transmitter and receiver at each end and some way of of mixing the signals so that they can be injected at both ends.

Table 1. DTMF tone pairs including the seldom used A, B, C and D tone pairs.

         1336 Hz  1633 Hz  1209 Hz  1477 Hz 
697 Hz      1        2        3        A 
770 Hz      4        5        6        B 
852 Hz      7        8        9        C 
941 Hz      *        0        #        D


Choose, say, row 1 (1, 2, 3 and A) for signals to the garage and row 2 (4, 5, 6 and B) for signals from the garage.

Links:


Radio Electronics is a blast from the past but looks as though it gives a very good introduction to the topic.
Intuitive Circuits gives an idea of the type of DTMF modules that are out there. I know nothing about them.
2-4 Wire Converters / Hybrids for telephones may be useful.