One cultural similarity shared by the aztec and incan civilization was that both -

Developed advanced mathematical skills.

Explanation:

The Aztec and Incan Civilization was part of the ancient civilization that flourished before the arrival of the European in the New World after the discovery of America by Christopher Columbus.

Both of the civilization cultures developed complex mathematical systems and impressive advances in astronomy.

The Aztecs had their form of calculation where they used a base-20 number system.