Ok, first things first... I was / am bored so am faffing about and thought I'd share what I found.

29x2.4 ardent, on a dually rim is 64mm wide, accross the carcass, knobbles are slightly less wide at 61.5mm
It does fit in the back of my large el mariachi with approx 4mm room on the chainstays to the knobbles ( chainstays are narrower at the knobbles).
Dia, approx 753mm

2.4 ardent on an arch ex is 59mm carcass, 61 mm knobbles. Obviously it fits in the el mariachi
Dia approx 749mm

Both fit in my kona raijin too, but with a scant 1.5mm clearance on the dually. It improves slightly at max chainstay adjustment. But not loads.

For reference, a knard on a dually is 75mm wide but also tall.
Dia approx 766.

I'm not sure I have a point but, I measured so thought I'd share

*edit, nearly forgot. A knard on a dually does not fit a 2012 reba, a 2013 sid, or an on one monocoque fork

So is an Ardent on a dually worth the weight gain over an arch? Particularly considering loss of mud clearance.

Humph, an actual intelligent and Relevant question!!

Err, I've been riding about 18 to 20 psi on the dually / ardent.
I only ever went down as low as about 25psi on the ardent / arch.

It feels slightly better over small bump stuff, vibratory terrain you know. But it's slight, and the rest if the time it's... Well... Dunno...

I built the wheels ready to get a krampus or something, but M now going to be waiting on a rooster, otherwise I'd have stuck with the arch. For cost reasons more than anything.
In fact, if I were buying now I'd get a wtb st wotsit rim for 32 quid, not an arch for 62 or dually for 92. If I wasn't planning on a knard that is

Although... It looks better on the dually

At Bespoked, Shona & Rich's tandem was on the 18 Bikes stand - it had 2.4" Mtn Kings on Dually rims and it looked vaguely exciting. Don't care what size they came up at, they just looked right

The wider rim for the win, as long as you are not racing. Creates more tyre volume, meaning less pressure, resulting in a more comfortable ride, esp over longer distances.