Can I apply this over a deck that I put Thompson wood seal over 2 years ago?

Thanks for asking a question. To determine if the surface is sealed or can be sealed, perform a "Water Test". The water test is accomplished by dripping some water on the surface of the substrate. If the water penetrates into the surface and makes it noticeably darker, then it is ready for another coat of Thompsons or another product. If the water beads up on the surface, then hold off on the application (currently the substrate is sealed and does not require an additional layer of sealer). Respectfully, Reggie