I think its wonderful that you want to shoulder some of the nursing responsibility and enjoy the bonding that nursing provides. This is a different situation than adoptive nursing, though, and its VERY important, as previous posters have mentioned, that your partner do all the nursing at least initially, to establish a good supply. Beyond that, you can be used much like a pacifier. In situations where women have adopted and nursed their children, and successfully established a milk supply, they have done the majority of nursing, which is what causes a supply to establish. In your situation, the bio mom is available for the nourishment, and trying to establish 2 nursing supplies with 1 baby might be asking for trouble.
As for the pshychological aspect, I certainly don't think it will be harmful. No different, IMO, than 2 parents feeding the baby a bottle, which happens all the time in bottle fed infants.