Here is picture of B747 SCA of NASA and the AN-225 SCA. There are additional vertical stabilizer in the tip of the elevator to the Boeing's, while the AN-225 is designed with the vertical stabilizer located at the tip of the elevator. As we know, AN-225 was also designed and intended to carry Soviet's Aircraft. Then my question, why do the B747 of NASA Shuttle Carrier Aircrafts (SCAs) need additional vertical stabilizer?
The AN-225 was designed primarily for carrying the Buran, so they didn't have to make structural changes to an existing design to accomodate the carrying of the shuttle. The aircraft was purpose built for that mission (although it never really saw much of that in practice).
The NASA 747 on the other hand, is a stock 747 that they added the ability to carry the Space Shuttle on its back. They couldn't remove the vertical stabilizer on the 747. Due to the shielding effect of the airflow when the shuttle is attached, they needed additional yaw control that required them to add the outer stabilizers.
The 747's used by NASA were converted passenger aircraft. The first one, N905NA was acquired from American Airlines made in 1974, the second one N911NA was bought from Japan Airlines, built in 1973 and acquired by NASA in 1990 or 1991.
There is an additional, possibly bigger reason than blanking of the tail by the shuttle. It's the same reason finlets and ventral fins are added to seaplanes. The side area of the shuttle ahead of the vertical aerodynamic center of the mothership is significant, and has the effect of reducing the weathervaning ability of the 747's vertical tail (imagine making the front arrowhead part of roof top weathervane larger; its weathervaning tendency is reduced).
Float planes have the same problem because most of the side area of the floats is forward due to the profile of the float's hull. I've flown float planes that didn't require additional fin area aft when floats were installed, but should have. You really notice the reduction in yaw stability.