How to construct a transaction with multiple inputs (joint payments)?

Question

I sometimes encounter Monero transactions with more than two inputs. If I have access to multiple wallets, how do I construct a transaction with multiple inputs from different wallets?

Answer 1

I sometimes encounter Monero transactions with more than two inputs.

This is very common and happens automatically. Your wallet uses outputs it receives, as inputs to fund a transaction. Sometimes your wallet will need to use multiple outputs (as inputs), to fund a transaction. This however is different from your question:

If I have access to multiple wallets, how do I construct a transaction with multiple inputs from different wallets?

To send/join funds from different wallets you need to first send funds from each of the wallets in question to a single wallet that will do the ultimate spending. This could also be a multisig wallet if there are more wallet owners than just yourself.

A couple of guides:

• https://monerodocs.org/multisignature/
• https://monero.stackexchange.com/a/5647/7493

Answer 2

It is theoretically possible send a transaction using outputs from different wallets, but there is no tooling for this currently. The idea is that each participating wallet will sign only the rings for the outputs they are spending. There is a proof of concept for this in https://github.com/moneromooo-monero/bitmonero/tree/multi but it turns out the mapping from inputs to outputs can't be made private, so it did not go any further, though I believe it works if you don't mind that drawback.

So AFAIK all transactions currently on the blockchain with more than one input spend from the same wallet.