Two-factor authentication Multiple devices

Most websites (that I've seen) that implement two-factor authentication do not support multiple account-related two-factor authentication features.

The main reasons I can think of this are the added complexity of the two-factor authentication system and performance considerations. But I'm not sure that there are security issues.

Are there security issues to allow users to have multiple two-factor authentication devices related to their account? Or is it simply a problem of performance and complexity?