user130868: Say I'm creating a multi-sig wallet with a friend to bet on the election. Can my deposit to the multi-sig wallet be dependent on the my friends deposit to the wallet?
Say I'm Alice, and my friend is Bob. Our shared multi-sig wallet is S. Would the following work?
Block N:
TxA
  Input1: A.amountX
  Input2: B.amountX
  Output1: S.amountY
  A.sig
  B.sig

Edit: Received good answer below, to summarize use Partially Signed Bitcoin Transactions (PSBT) BIP-174, it relies on Sighash Flags, this enables parties to sign more than just their own input.

user63872: To accomplish this you could create a single transaction that you both supplies inputs to, and that creates an output paying to your multi sig address. In order for the transaction to be valid you will both need to sign it, and of course neither of you should sign it unless the other has supplied an input with the agreed upon value.
See: BIP 174 Partially signed bitcoin transactions

130868: <p>Say I'm creating a multi-sig wallet with a friend to bet on the election. Can my deposit to the multi-sig wallet be dependent on the my friends deposit to the wallet?</p>
<p>Say I'm Alice, and my friend is Bob. Our shared multi-sig wallet is S. Would the following work?</p>
<pre><code>Block N:
TxA
  Input1: A.amountX
  Input2: B.amountX
  Output1: S.amountY
  A.sig
  B.sig
</code></pre>
<p>Edit: Received good answer below, to summarize use <a href="https://github.com/bitcoin/bips/blob/master/bip-0174.mediawiki" rel="nofollow noreferrer">Partially Signed Bitcoin Transactions (PSBT) BIP-174</a>, it relies on <code>Sighash Flags</code>, this enables parties to sign more than just their own input.</p>
 63872: <p>To accomplish this you could create a single transaction that you both supplies inputs to, and that creates an output paying to your multi sig address. In order for the transaction to be valid you will both need to sign it, and of course neither of you should sign it unless the other has supplied an input with the agreed upon value.</p>
<p>See: <a href="https://github.com/bitcoin/bips/blob/master/bip-0174.mediawiki" rel="nofollow noreferrer">BIP 174 Partially signed bitcoin transactions</a></p>
