I've not found a way that Resolve can do this. I tend to use multichannel files from the external recorder, but if you are presented with separate files, you will need to create multichannel files first.
If you are familiar with the command line, you could try getting and using the command line utility "sox" that will take your separate WAV files and create a multichannel file which you can then sync in Resolve.
e.g.: I have the following:
Scene_12D-T002_Tr1.WAV
Scene_12D-T002_Tr1_D.WAV
Scene_12D-T002_Tr3.WAV
Scene_12D-T002_TrL_R.WAV
sox -M Scene_12D_T002_*.wav Scene_12D_T002.wav
This will create the required multichannel file, but note that you will lose your metadata.
Declan Smith
http://www.madpanic.tv
Davinci Resolve 15.2 / Reason 10 / BlackMagic URSA Broadcast / Zoom F8 / FCPX / Adobe CS6 / FCS3 / Roland R26 / Canon XLH1 / Canon 7D / Cubase
"it's either binary or it's not"