Can two people be on my sentence account at the same time

No, two people on the same account at the same time will cause one of them to log off.
However; Two people can play on your account if you allow the other person to access all your info but it has to be on different times.

Technically speaking, only one person is supposed to be on an account. According to the Terms of Service, only one person to an account, unless the other person is a minor child of the account holder.