What's the Name of That Book??? discussion

OK, here's another one I can't remember. Might be a 3 book series.
Took place mostly on another planet. A U.S. Earth man and woman are captured by aliens. The aliens repeatedly clone the two for different experiments. Story is told from the man's viewpoint. The couple fall in love. They are kept in a very simple room. I think stuff that drops on the floor transfers through it. At one point the aliens are surprised to learn that human waste has living bacteria in it. They had disposed of the waste in the planet environment, but were able to recover/destroy it before it corrupted the ecosystem. The man learns that some of his clones have been designated TTD, 'test to destruction'. Near the end, several pairs of the clones have returned to Earth. They live together or in a small set of dwellings. The males give themselves different appearances by varying their facial hair, one has a moustache, another a beard, another none and so on. The clone that we have the viewpoint of thinks he should be matched with the original of the woman. He meets with the President of the U.S. who gives him advice and he decides to stay with his clone female.
I want to think it was written by a well known, long time SF writer. It isn't Isaac Asimov or Robert Heinlein as far as I can tell. I read the book(s) around 20 years ago as best I can remember.

The Eschaton series by Frederick Pohl has been suggested to me on another board. Looks possible by description. I've got it coming from the library.

That does sound like a good fit.

Which other board?

AbeSleuths.

Abebooks.com - BookSleuth?

AbeSleuth and AbeSleuths turn up nothing on google.

Also, if your library loan turns out to be the correct book, you can edit the title and select the solved folder (but you probably already knew that). There's also a 'possibly solved' folder, which I tend to put these requests in (when I think it's a good match).

Yep, Booksleuth on Abebooks. Sorry about the conflation.

And it's the correct answer. The Eschaton series by F. Pohl.

ETA: First book in the series is The Other End of Time by Frederik Pohl.