If nujacks were into Testament during Low(which I believe was their biggest seller)
Not that it matters, but I believe Practice was their biggest so far.
There seems to be a little inconsistency in your post. You say that at this point Testament is drawing the same crowd that Exodus and then you justify the statement that Exo should headline by saying that "giving Testament bigger billing and advertizing will draw attention away from Exodus...putting more emphasis on Exodus right now won't hurt Testament as they are already well established and Exodus needs a bit more of a boost...".
So are they drawing the same crowd and are equally popular or are they not? Make up your mind, 'cause there's an incoherence here.
Besides, being a "senior" doesn't necessairly imply the privilege to headline. Or are you trying to suggest that every band that's older should automatically be 'above' the younger bands in the billing? That would lead to really odd orders of performance during most festivals and many tours. Where's logic here? That's not a good argument.
Anyway, just giving my two cents. I'm not trying to say TestAmenT should headline during the, for the time being virtual, tour with Exodus just because I like them more. I'm trying to to think the way tour organizers\promoters think- who sells more, draws more people to the show and that= more money. And TestAmenT has sold much more records (and the fact that Exodus have the new one out doesn't overshadow it). That's a fact. These are the rules of this business. I'm not trying to say they're just or that I support them, but they seem logical and it's the way things are and we can do nothing about that.
These are all just hypothetical divagations and unproductive, though. If such tour was to happen (I hope and I doubt it will) the bands would probably co-headline (because of the mutual respect and friensdhip, I dare to guess). It just wouldn't feel right for anyone if one of them was just supporting. But if there really HAD TO be some hierarchy, TestAmenT would be higher.