Can a diplexer connect two antennas with 2.4GHz signal to one transceiver?
A diplexer combines or splits significantly different frequencies. 'Significantly' means far enough apart so that they can be separated by a filter. Where the two filters join, each is high impedance at the other's centre frequency, so they don't load the other signal. If you can build the filters, you can make a diplexer. If you can't, then you get a signal combiner/splitter, with at best 3dB loss.

All nominally 2.4GHz ISM band signals, mice, WiFi, bluetooth, are essentially the same band. It's not even possible to specify a filter to split them, let alone realise one in hardware.

2.4GHz and 5GHz is a relatively easy split. I've been peripherally involved with diplexers that split 1800 from 1900 MHz mobile comms bands, and they used some quite expensive engineering to the get the Q and stability required.

There are several ways to approach a signal combiner design. At these frequencies, the most practical would be a Wilkinson combiner, using microstrip on PCB.