Are you certain that the ceramic coating on the merged header will hold up after heating it up cherry red? Just wondering...
Oh hells no it won't survive.
But I guess you could say the integrity of the finish is beyond the point. If the joint won't come apart then the header is junk anyway. It doesn't cost much to have a header re-coated.
If it comes to that I do feel for you though. It's a real bummer to have to willfully destroy something just to save something else. It's sort of like waking up the day after a party with an ugly girl laying on your arm. You know you have to chew it off to keep from waking her.