If you're using the TS SDK, it might automatically use all your coins as long as they're not engaged in the transaction. It can be challenging with exactly 2 coins, but if you set it up correctly, it should work. Despite initial difficulties, it seems that once you try again or follow the correct process, the sui client merge-coins function can work effectively.

Gas coins are automatically merged, so if you execute an empty transaction with your coins used for gas, you will end up with a single coin. The SUI client (CLI) only lets you specify one Coin to be used for gas. You might try executing without specifying any coin, or make sure the transaction automatically computes the necessary coins.