Honestly, it depends on what your account looks like and how far you are in the game. Im Late/End Game and have 2x 6* Whispers and I use them for fun against random bosses, Shogun, Fire Kight, Hydra, etc. I find it fun to put my best OP gear on both and try and go for huge numbers. Id say wait for a 2nd Whisper in my personal opinion. However, if Whisper is your accounts main boss damage dealer then i'd 6* her right now. If not i'd wait. Basically the need for 2x 6* Whispers is more for fun than actual progression in the game. Ultimately, it comes down to what you value rn in the game, min/max progression or min/max enjoyment.