I have two tag groups (‘Grade level’ and ‘Subject’) and I’d like to make it so a user needs to select one tag from each group when they post in this particular category. I’m looking at the option below:
So, I think you can do this by setting various scattered configuration options, as long as you can accept the limitations.
On the top-level Tags page, go to Manage Tag Groups and change “Grade level” so “Limit one tag per topic from this group” is checked.
On the category’s configuration, on the general Setting" page, change “Minimum number of tags required in a topic” to 2.
On the category’s configuration page for Tags, restrict tag groups to just the two, “Grade level”, “Subject”
Require 1 tag from “Grade level”.
Now, users will need to select a grade level tag, and then also one other tag — and Subject will be the only other possible choices for that other tag.
This doesn’t work if you need to tag some posts with multiple grade levels (maybe in that case you can switch which is which — allow multiple grade levels but only one subject), and it doesn’t work if you want to also allow other tags.
Another approach might be to use the Parent Tag functionality (back on Manage Tag groups) and require the child tag group for the category. I haven’t used that, though, so I’m not sure if that works. If it does, I think you could avoid the limitations (at the expense of possibly allowing nonsensical combinations after your first two tags).