So, I don't know if this happens in Vanilla, because there is a distinct lack of paired techs in Vanilla, but in DE I noticed that it looks like the cost requirement for both techs in a pair must be met for me to research either one.
In this shot, you see I am attempting to research Battlefield Triage for 300 food and 200 glory, which I have enough of, but the game says I have insufficient wood.
Here, I am attempting to research Sluicing, which costs food and wood, but the game says I have insufficient metal.