Ashton, Yes, you can belong to more than one project. For example, someone who is interested in the Puritan Great Migration project may also want to belong to another project because an ancestor migrated West on a wagon train. The problem is that you may feel that you are not able to give enough time to a project.
Remember, you don't have to belong to a project to contribute to the profile of a person who fits in the project. I find that it is probably best to join only project when you are willing to spend time on the project.