I'm assuming you mean feeding the blank over the lower platen, placing the transfer and pressing. I do this all the time for thick fabrics and to press multiple transfers on one shirt. You can do this with anything that FITS. I have a 15x15 clamshell and can feed all my sweats, hoodies, and adult t-shirts without any issues, but once I get to children's sizes, no go. A 16x20 might create more issues with some of the smaller sizes.
You can still do transfers on both sides of a shirt by using protective teflon sheets.
You can still do transfers on both sides of a shirt by using protective teflon sheets.