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they each contribute the seed they already have
Do not share your seed phrase. Do not! Instead, the xpub should be shared. Download sparrow, create a multi-sig wallet and it will be immediately clear what I'm talking about.
now they have an extra multisig / shared wallet on top of their own solo wallet that they can still control as normal?
Yes, they would have a wallet that's single sig and a wallet that's multi-sig with different Bitcoin addresses. If you download sparrow wallet and create a single sig seed phrase for one script type and then create a new wallet, import the seed phrase and use the multi-sig script type, you will see different Bitcoin addresses and see what I'm talking about.