How can the Father and Son be separate “persons” when…

How can the Father and Son be separate “persons” when the Father dwells in the son, and the son in the Father.

If “For in him dwelleth all the fullness of the Godhead bodily” (Col 2:9) and it is only the Father that dwells in Jesus (John 14:10)
How can the Godhead be more than one?

If Jesus isn’t the Holy Spirit, why does he say I am with you always till the end of age?(Matthew 28:20)