I know this is my first post here and some may question the credibility of it, but if you head over to the show-setlists forum on AbsolutePunk you will see I announce tours weeks and months before they are announced a pretty consistent basis.
With doing this, I have a "respect" thing i go by. If the band isn't currently out on a tour or has one coming up before that tour, I will spill the beans (see: Bayside/Silverstein co-headlining tour this spring, Current Parkway Drive tour, Between The Buried And Me/Job For A Cowboy)
I can say the two bands co-headlining this year's summer slaughter HAVE been mentioned in this thread already. Some could argue it is a co-co headliner with the third band that is confirmed to be playing. They are a pretty well respected band, but it is being billed as a co-headliner from my understanding.
Hopefully I can post more about this soon! Don't mean to tease you guys. You might be able to figure it out. Both co-headlining bands are good friends.