No, you can only select 1 target field at a time.
There are a few special "Fields" like _ALL or _TAG, but you loose control over what fields will be changed.
You can also program using the «Actions», command in Mp3tag, thanks to @ohrenkino and @LyricsLover for showing me, how to program with this command.
The two tags: | START DATE | END DATE | change «-» by «/»
You can program your 2 tags semi-automatically with Mp3tag «Actions», after programming, you can execute the «Actions» command for your 2 tags with just one mouse click.
Just a hint: If you want to have more or less the same action only for different fields, it would be easiest to copy (there is a button for that) the action and set the new field name.
Everything else could stay the same.
Thanks @ohrenkino for this fairly detailed information. @LyricsLover also wrote me some examples.
Ohrenkino, I'm working on some of my tags with ('Album') ('EP') ('LP') ('Single')
If one day I need to program this type of data with Actions, I'll run some tests. If it doesn't work, I'll write on the Mp3tag Community to see if we can program them.
Thanks for your help, Ohrenkino, and the other volunteers too, and Florian, who gives us great updates with new features that sometimes impress me...