Wouldn’t it be convenient to have a combined Greek NT and LXX? Well, last year the German Bible Society published one. You can buy it on Amazon here: LXX-NA28 Greek Bible (Biblia Graeca). It will cost you a pretty penny, though, over $100. See Abram K-J’s post with details here.
Of course, with smartphone technology you can get apps with LXX/NT, or just browse to a site like https://www.academic-bible.com/en/online-bibles/septuagint-lxx/read-the-bible-text/ where you can read the LXX or NT.
If you don’t need the critical apparatus and you don’t mind reading the traditional texts, you might want to check out the Zoe Brotherhood‘s Greek Bible, which has LXX with apocrypha and NT in one volume–and it actually looks like a Bible. The font is readable, the pages are heavier weight, and all the book names/numbers are in Greek. Best of all, you can get it for about $30. If, that is, you can find it. I lucked out with my local Greek Orthodox church’s bookstore, so you could try them. Others have had luck with Holy Cross Bookstore in Massachusetts. If you’re real adventurous, you try the Varnavos Foundation, though shipping costs may kill you. Also, Christianorama seems to carry it.
Comment below if you’ve found a better edition, or have found a good place to buy the Zoe Brotherhood edition.