I too keep two copies, as well as the originals - on different brands/manufactured discs if possible.
The originals - “everyday use” discs - are stored in a box, in plastic wallets, one set of back ups is stored in slim jewel cases on a shelf, in a cool dark room…and the other set of backups is stored at my mum’s house, in wallets, in a box.
They all seem to be holding up very well, even the “everyday use” discs, since I switched to quality media (thanks to the guys here at CDF ).
Oh, and all discs are stored vertically…not on top of one another.
No idea if this is the best way, but as I say, it works very well for me!