It is quite simple really. I bought a three pole double throw switch (3PDT) which can switch between 2 sets of 3 wires. Left, right and ground. I bought 3 - 1/4" headphone jacks and the hard part for most, 3D printed a case. 3D printers are amazing by the way. One could very easily buy a small plastic project box and drill holes for the switch and jacks. Solder them together and boom, you have an easy way to compare two setups with one pair of headphones.
I can connect different sources like computers and DAC's or DAP's like the ones from FiiO. Play the same song on both synchronized. Then match the volume on both of my AMP's and switch between the two and see if there is a difference. Surprisingly, I don't find much difference in DAC's or DAP's. There is a big difference in AMPs and headphones. Having two identical AMP's helps with comparing DAC's / DAP's.
I hope that helps.
Mike