Here is a photo of the inside of a high/low pass mobile duplexer I built many years ago. Some people may call this a diplexer but it performs the same function as a duplexer.
It was built inside of a Hammond 7.25" X 4.5" X 2.5" cast aluminum case. The internal partitions were constructed using a cut-up Hammond aluminum chassis.
The antenna port (top) feeds 2 parallel tuned circuits, one tuned for 2 meters and the other for 6 meters. The parallel tuned circuits feed a high or low pass filter. The high and low pass filters feed 2 and 6 meter radios via SO-239 connectors. The 2 meter parallel tuned circuit feed the 6 meter low pass and the 6 meter parallel tuned circuit feed the 2 meter high pass filter. The 2 meter high pass is shown on the left and the 6 meter low pass is shown on the right.
The antenna used was a 2 meter Larson 5/8. The antenna acted as a 1/4 wave on 6 meters. The antenna had a floating coil at it's base, that is, it was not tapped down. All 2 meter 5/8 wave length antennas may not work in this situation. The Larson antenna provided a good match on 6 meters.
I got the high and low pass design information from one of the The ARRL Handbooks. If I were to build the duplexer today I would use software available for that purpose, it's just easier.
Here are the measured test results:Insertion Loss at 145 MHz (Ant. to Radio/Radio to Ant.) 0.4 dB
The duplexer provided enough isolation so that no desens was present on either the 2 or 6 meter receivers when one or the other radio was transmitting. The transceivers used at the time had good receiver front end selectivity. I am not sure if most of today's VHF Ham transceivers would perform as well given that most seems to be able to receive from DC to light.
I don't know the maximum transmitter power the duplexer will handle. It will handle 50 Watts on 6 meters and I think I have tried it at 100 Watts. On 2 meters, the maximum power I had available at the time was 30 Watts. I do not operate on 2 or 6 meters much anymore and I have not used the duplexer in a long time.