All you have to do is buy a BFA license on a second account on your B.net account. Example, you have BFA on account 1 but only Legion etc on account 2, buy BFA and active it on account 2 and now you can use the 110 boost on account 1. Tested since Legion and still works.
Also if you have too many wow accounts on your b.net account, just ask blizz to remove some from your b.net.
Buy it for 28 Euro here: https://www.cdkeys.com/pc/games/worl...w_aref=reguser