Go to the BP_MasterMeleeWeapon and change the FP Melee Montage and TP Melee Montage variables to arrays. Now create a new function called Find Melee Animation and add 2 outputs to the function called First Person and Third Person and set there variable types Anim Montage.
Next copy the image below.
Next go to the Multicast Melee Swing event and add 2 new inputs called First Person and Third Person variable type Anim Montage. Then connect them up to the collapsed nodes like shown in the image below.
Then go in to the Multicast Melee Swing Events and copy the image below.
Then add the Find Melee Animation node like shown in the image below and connect it up to the Multicast Melee Swing event.
Next you will need to go to all your melee weapon holdable blueprints and set the melee montages again. You will now be able to add multiple montages.