? Split manifold style headers for XJ ?

Does anyone know of a header manufacturer that makes a dual exhaust manifold for the 4.0 liter Jeep 6 cylinder? This would allow a true dual exhaust system.

I recall that this used to be custom-done on in-line motors years ago with (do I remember right) like the Chevy 292 sixes.

Or, maybe it's just not an efficient exhaust setup.

Thanks!

MikeinIN