Possible to add custom object row before lead row via SOAP API?

Is it possible to add a row to a custom object before the coresponding lead record is created?  Since we set the foreign key to links the two both when we add the custom object row and the lead row, it would seem this would be the case.

For example, if the lead table has the field "company_user_id_co" and the custom object has the exact same field.  We are telling Marketo what the value should be in this column on both records, so the insert order should not matter, correct?

If we did send the custom object first, would is still trigger the "Add to CO" in the UI?