Actors booking more than one project at the same time is not uncommon. The term ‘first position’ refers to whichever project is your number one obligation.
Obligations are typically determined by a first-come-first-serve basis. For example, if you first close a deal to work as a guest star, and then book a role in a feature film, if there are any conflicting work days with the two projects, the guest star that you booked must be given top priority, i.e. first position. Deciding which projects should be first position should always be done after careful consideration and consulting with your rep(s).