Tetris Effect: Connected

When looking at this achievement, I was under the impression that you had to use two consecutive straight pieces to get a back-to-back; however, it seems that you can place pieces freely as long as you don't clear any lines. That being said, you basically just have to set yourself up to get a Tetris 11 consecutive times (10 following the first one since a back-to-back can only happen on the second) and only clear lines when you can get a Tetris. Doing this on one of the slower-paced game modes will undoubtedly make this easier as you'll have an easier opportunity to set everything up. Also, don't forget that you can use to save pieces, which seems to be best for keeping a straight piece on-hand.

Helpful tip: Try Quick Play under the Effect game modes. Change your goal to 60 lines and make the pieces fixed so you don't have to rely on luck.

tl;dr set yourself up for Tetrises and only clear lines when you can get a Tetris. Do that until you get 10 back-to-backs.

Credit to Grozdof-Games for the video: