Is it alright for two people to simultaneously be logged into a WP site as administrator?

If you are using a newer version of WordPress, you’ll see that you can’t both edit the same page/post because it will show that as locked (you can take over though). I agree that a separate admin account would be good. WordPress is designed to support many users logged in and active. The only issue becomes if you are doing administration such as settings/tools/widget/theme modifications where actions can be overridden by the other person. I would suggest an open line of communication as you work. In a development environment, the same applies if you have multiple hands on the same project and can’t resolve in a local environment with SVN/Git control.