Burn two NFTs to mint a combination of the two. Has it been done? Don't know where to start

I'm the creator of emoshuns. The contract is a basic erc721 contract that looks similar to the vast majority of any pre-generated NFT project contract. I would like to offer the ability to burn two emoshuns to create a combination of the two. I don't know where to start in terms of writing such a contract. Any projects you know of that did this? Would love to see some other contracts that solve this problem. Thanks in advance!!