slide show using more that 1 album

If it's the albums in your local library try this:

1 - create a smart album with these criteria:

If you don't have any animated gif images then ignore the animated portion.

2 - Control (right) - click on the smart album and select Create ➙ Slideshow ➙ Photos from the contextual menu:

Opps. What I posted earlier will get ALL of your photos in the library, not just your selected albums.

You'll have to add the albums individually to the smart album like this:

Create another album with photos from both cites.

Are you asking about standard albums for a slideshow in Photos for Mac or a shared album slideshow on your Apple TV?