Will buying two Lizard Minion Repellants and one trap give you a 100% change of getting the item in the lizard minion event? Or can you only buy one of each?

You can have each, but I think you can still miss the lizard, but the chances are much better with both.

You can use both.

Personally, I've never missed an item when I've had both in my inventory. Some people say it's not 100% though.

Also never missed an item with both in my inventory.

I have both the spray and the trap, also never missed an item.