I think my title contains the complete question, but I'll try to rephrase it here. When playing Canasta, if the discard pile has already been frozen with a sideways facing wild card, can an additional wild card ever be placed as the current discard? I don't believe that is allowed, but if I am wrong and you are allowed to discard a second wild card, can the pile then be picked up by the next player displaying two wildcards for a meld?
You can always discard a wild card even if one is already on the pile. It freezes the pile for both sides, much like the start of the hand.
When the pile is frozen, it can only be unfrozen when a natural (non wild or 3) is on top of the pile and it is taken with a natural pair of those same cards.
I've played several different variants of canasta, but the above rules are the same.
Here is one set of rules for Canasta, I'm not aware of an official set of rules from a universal governing body. https://bicyclecards.com/how-to-play/canasta/