I think a part of the problem with this discussion is terminology... a header tank, by definition, is at the "head", or top of the fuel system.  It is above the level of the mains and needs to have its fuel pumped into it from them. An auxiliary tank at or below the level of the mains that receives its fuel from them by gravity should be called a sump tank, or by some other name that differentiates it from a header and explains that it functions more or less as a sump for them.  The sump tank needs to be vented to (or through) the mains as its ultimate level is determined by its relation to them, as opposed to the header which must have fuel lifted to it from the mains and can be treated as a free standing 3rd fuel tank.  Just my 2c.

 <Marv>