Ahh! OK so you have a separate o/p for each impedance! Well I suppose you could just parallel up the the 16 ohm so that you'd have 2 16s in || for 8 in total but that isn't going to be very usefull if you change cabs in the future.
I have a Trinity 18 combo and it only has 1 o/p. I built up a little plastic box with 2 o/p jacks in || and a short wire with a 1/4" plug on the end. I plug it into the single jack and plug the speakers into the box. I'm going to build another one that puts the 2 cabs in series. Maybe a series/parallel switch
That's all I got
Stew
PS I was just thinking..and maybe phil or Stephen will chime in..what would happen impedance wise if you just used one of the other o/p taps for the other cab?!