Can you explain in a little more detail how this done ?

might be different for 928 due to the oddly shaped heads (the bit that sticks forward on the LHS head) but on the DOHC 944 engine you put 2 exhaust cams in the head (since exh has a snout for the camshaft belt gear), fit cam gears to each cam and get another belt to fit.

2 real issues - finding a gear with few enough teeth to work. 944 has timing belt teeth 3/8"/9.5mm spaced apart, times a given number of teeth makes a circumference from which you can find diameter of the gear.
you can only fit a pair of gears so big due to the width of the head (spacing between camshafts). 928 head is the same width so same space constraints.

however if you look around a bit you find that Honda B series uses the same belt pitch and they came with cam gears with 34 teeth, this circumference fits perfectly on the 944/928 head widthwise. need to enlarge the center bore on the Honda gear to fit the Porsche camshaft. also, to match the 34 tooth cam gears, the Honda crank gear is 17 teeth, and it's nearly a perfect fit dimensionally for the 944 crank snout.
the 928 snout is much different so you'd need to do some machining but nothing super complicated.

the other issue in the 944 is due to the 30* canted angle of the engine, the "2nd" gear bumps thru the hood requiring a cutout.
this should not be an issue on the 928 because you have 45* cylinder banks and MUCH more room under the hood.

http://forums.rennlist.com/upload/powerhaus_34.jpg