No, you con not import or combine multiple libraries within Photos app. You can set, open and work with multiple libraries into Photos app. When you first time launch Photos app, it will ask you to choose which library would like to import into Photos app. Once you choose your library, you can not import another library into Photos. That means, if you choose iPhoto Library while launching Photos app, then you can not import an Aperture library into the Photos library. But, you can create a separate Photos library and import your second library into that. To do so, hold down "Option" key when launching the program.
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