There are a few different variations of this question, and we'll cover them all in turn.
Can I log into the same computer with more than one account?
Yes. More than one user can use the same computer to ride.
Can two people use one computer at the same time?
No. Much like other MMOs, Zwift does not support local multiplayer. Each user will need their own computer.
Can I log into two different devices with the same account?
Yes. Zwift stores your account data on our servers, so you can log in anywhere Zwift is installed. This includes PC, Mac, and iOS, allowing you to use up to all three with the same account.
Can I log into two different devices with the same account simultaneously?
We do not recommend this. Logging into two different devices with the same account simultaneously can cause a number of issues including poor performance, forced log outs of your other session, and old data overwriting new progress. So don't do it.