A 2-3 multisig needs 3 seeds (private keys), you can reuse the seed you used for singlesig, as one of the 3 seeds.

Your funds are seperate from eachother, because your singlesig generates a different public key (and thus addresses) than your multisig.

As an example, when you have a 2-3, with 3 seeds, A, B, C... You could switch one seed for another: A, B, D.

This now gives a totally different public key and addresses, so basically a different wallet, while A and B are reused. Note that this is just an example, not that it's smart to do this last example 😂