I don't need one myself, but I bet it a part you might be able to get from Radio Shack.
You can indeed run the two 16's into the 4 Ohm jacks. And you will hear a sound difference. I don’t remember what it is, but someone here will be able to say. They might even know if using the box reduces the tonal change (something I’ve wondered). If it doesn’t really do anything beneficial I’d skip it (reduce parts of the signal chain).
It sounds like most people having an impedance mismatch generally just compensate for it via the amps jacks.
Strat
Ps – tks for reading the manual. Your already way ahead of a lot of people.