Generally speaking, you can display all of the ancestors and descendents of any one person in the tree. You can often display everyone in a relalitvely simple tree by clicking on the "correct" person.
Additionally, there are some options that you can set that determine how "far" from the clicked person people will be displayed. To set this, click "Show options" at the bottom of the screen and set Parents, Children, and Others to their max values.
This is a pretty generic answer, so if you try this and still can't see both people on the tree at once, it might help to report back and tell us how they are related.
