Seed investors are primarily evaluating the team and the space in order to make an investment decision. Having an unknown co-founder they would need to work with in the future is a red flag. If you still intend to do that, be up-front and work closely with them in finding the right person. This is important for dilution in future rounds. You want your investors to share in the dilution if you bring on a co-founder later with material amounts of equity instead of diluting just the current founder(s) share.
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