Thanks Kappes.
Again, it seems where I went for the files didn't have the LATEST version as their latest. I have downloaded it but unfortunately it looks like multi tags weren't supported in this edition either.
Looks like I'm SOL on this one...
EDIT:
Hang on though, If i just make sure that both control sectors' A160 linedefs have the same sector tag, that should work, right? I'm so new to 3D floors, slopes and gzdoom (up until not long ago, I was running DCK in DOSBox and thinking I was pretty nifty for getting that working again

) that I can't remember all the stuff I've read over the past week.
If this is true then:
a) I should still be able to put slopes on any 3D floors I want,
and
b) It does mean having to manually change the tag when I add the second (and third, etc.) 3D floor. Unless there is an option somewhere to do this for me? If not, could there be? Probably a lot of work for just my benefit, but perhaps a plugin version for matching the rules for GZD 1.8.10 and earlier would be used by many?
I get that this could all probably be done easily with ACS, but that's probably my next step once I understand all of this properly
