Currently it is not possible to create a custom action that is only shown when more than 1 item is selected.
I agree that this would be nice to have .. bug with proposed patch for an old thunar version already exists. Please feel free to test the patch and port it to master, if you can ! ( Review/apply the patch is on my personal TODO ... but so far misc other stuff was more important ) *** This bug has been marked as a duplicate of bug 7353 ***