Even though he didn't have a twin to practice with, he had the best next thing.
Thanks to Solus's Monocle form, he and Tista could share a perfect mind link. On top of that, as long as they were inside the tower, the power core relieved them from most of the burden that the artifact inflicted upon their minds.
After teaching Tista Xenagrosh's technique, Lith had devised for his sister and himself the following routine. While he trained, Tista would study him with the Eyes of Menadion so as to spot his mistakes and keep his life force in check.
Once Lith's condition neared the critical point, she would stop him and then the three of them would revise the footage together. This way, Tista wouldn't make his same mistakes when it was her turn to practice and the siblings would both learn twice as fast than if they were alone.