Can a J-T combination be counted as a single honor under a "two honor" rule?
Broadly yes. A common take on this is ‘Two of the top three, or three of the top five.’  That means QJT counts (or KJT or what have you).  See for example this article on Fifth Chair.
Also note that this only really applies first or second seat, and some would even do it first seat Not Vulnerable. Preempts are getting somewhat more aggressive over time as their value is shown - some even do five card weak twos now, though I find that more risky than I can tolerate.