No way is Iron Maiden filling a 65,000 seat venue and especially not with Judas Priest. The bands have a massive overlap in their fanbase. I'd wager (having no reliable data whatsoever) that 50% of the audience that sees Maiden gigs also sees Priest gigs.
So, math time, last time Maiden played London they did 2 nights at the O2 Arena, which probably has a capacity of about 16,000 or so for a concert with a stage at the end? At maximum, that's a total of 32,000 people (and no doubt many of those were going to both shows so let's say....25,000 at a stretch).
Priest are about to play the OVO Arena, which has a concert capacity of probably around 10,000 or so? If they sell that out, we're looking at an average London ticket sales in the realm of 25k for Maiden and 10k for Priest. And no doubt half of the people seeing Priest are also seeing Maiden. So, let's give them a health average of 30,000 tickets for London.
Add on top of that all the people who would travel to London for such a special event, I'd say....maybe 10,000 people at most?
That's still a 20-25,000 ticket sales deficit.