Unfortunately no, they cannot be linked if they have different User ID's.

Ok, thanks. I should have also asked if a user ID can be changed so that the Mint and TT have the same ID (and can then be merged)

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@rhodion wrote:

Ok, thanks. I should have also asked if a user ID can be changed so that the Mint and TT have the same ID (and can then be merged)

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You cannot change a User ID on one of your Intuit accounts to a User ID that is already being used on another Intuit account.