I'm not saying that - I've noticed this about you already, you don't tend to read posts you reply to. You stated originally that Flat and Vee engines need separate pipes, and I just made it a bit more clear that not ALL do. Then you come across with a V10 and a V12 example, where of course it's more likely due to packaging.
Also, bear in mind that the English don't use the term headers as it's very very vague. We tend to use the term exhaust manifold, front pipe, primaries, and that sort of thing. As an American/Australian 'header' can be all of these things it's not really ideal
Assuming you mean header to mean manifold, it's fairly likely that the manifold on a V10 car will more that just a small section, but several feet of pipe so as to generate the correct mass flow rates and primary lengths. You might have crossover pipes, and 4 into 2 into 1's over several lengths, which COULD culmulate into a single pipe. As I say, it depends and you didn't make that dependance very clear.