Cartwright has NT potential, although it's long term and more as a specialist when the team need a SF with outside skills.
Sunbury looks a very good prospect for the U21s, but not the Full NT (because of his potential)
For Cartwright, you need to keep up a mix of outside shooting and pressure training (which will also suit Sunbury) and then work on his handling and driving when he's 22. The NT coach may also want more passing - but you can do that when he's older (23 or 24).
Two very good trainees who will train nicely together. Train one position and play them both in that position, starting Sunbury in the more difficult game of the week. When Sunbury hits 22, think about selling him and using the money to strengthen your team.
Hope this helps.