You can only have one project open at a time in After Effects.
The way you've phrased your question suggests that you may not be clear on the difference between an AE project file (project.aep) and a composition. It's the Composition(s) that you see as tabs in the timeline. One project may have one, or multiple compositions in it. If you're uncertain about this I'd strongly suggest you review a few basic tutorials at: https://helpx.adobe.com/after-effects/tutorials.html
However, you can import one project into an exisiting project. Simply by using the 'File > Import' menu option, or by double clicking in the project panel. This will import the project in it's own folder, with all the related compositions and assets. You can then double click one, or more, of the imported compositions to open them as tabs in the Timeline.