[NTG-context] header in the middle on doublesided layout

Hello, I would like to set a different header on left and right page, but I would like it to be centered on page. How do I do that? The command \setupheadertexts[a][b] puts "a b" on one page and "b a" on the other while \setupheadertexts[a][b][c][d] puts "a b" on one page and "c d" on the other. On single-sided layouts I could use \setupheadertexts[a] or \setupheadertexts[][a][] but using 6 parameters on double-sided doesn't work. I am now hacking with \setupheadertexts[\hbox{\kern-5mm\myheaderright}][][][\hbox{\myheaderleft\kern-5mm}] \def\myheaderright{\hbox to "textwidth+10mm"{...}} but that's a bit ugly. Mojca