It was previously possible to use a loophole to keep both Karlach and Wyll without him getting horns, but it involved killing Karlach ( as a party member ) and reviving her after Wyll's encounter with Mizora which is to my understanding fixed and not possible to exploit anymore.
There is however some conditions before this triggers and there might be one painful way to try and see if it actually works to keep Wyll hornless with Karlach in your party;
Before every long rest you do kill Wyll and then just revive him back to your camp ( or party ) the next morning and keep repeating this until you reach Act 2, apparently you can ignore Mizora if she comes roaming to your camp if Wyll is dead.
I would love to hear back if you want to try this odd method.
