Yeah, it's impossible to say one way or the other because the setup is underdefined and leaves a lot of room for ambiguity or loopholes.

On that note, don't beat yourself up or consider yourself stupid because of that. Even though it's questionable whether it would work or give them room to screw you, I think it was a good creative solution to the riddle that I've never seen before. If you came up with that on your own, I'd consider that a sign of good potential. Nurture and refine that, don't try to beat it down to avoid being wrong ever. (Haha I really hope you're not like 50 or that might come off as really condescending rather than encouraging).

Like, thinking about it more, I think it can be resolved by changing the "and" to an "or", at least on the lying side. Though that would open up the truth side to be able to sneak in a lie while technically telling the truth. But there might be another adjustment that would close the loophole entirely and give a solution that doesn't require a reference to the other guard's answer.