When using mismatched impedances between the amp and speakers there is a greater strain on the output stage. Total output power to the speakers is reduced, the tonal balance changes, and part of the output signal is reflected back to the amp. This reflected signal can exceed the voltage ratings of your output tubes and cause them to arc or short out - even worse the output transformer can be damaged. Try to keep the output below maximum, this will prevent possible problems.
Whenever you mismatch a tube amp with a much higher load (speaker) impedance, a higher signal voltage is produced across the primary of the output transformer. One loud popping note on the high E string and you could arc-over the output tube sockets, fry the tubes, or zap the output transformer. This is why you cannot run a tube amp without a speaker connected to it. If you must mismatch the load impedance use a lower than rated one, as you are doing now by connecting both cabinets.
Having said this, and making it very clear that I do not advise anyone to connect a higher than rated impendence to any tube amp, generally there is no problem when operating the amp at 8 ohms if your amp is well designed and in good working condition. Connecting a 16 ohm load to a 4 ohm amp is, of course, more risky.
As with all tube amps the correct impedance setting for the amp is the one that is the same as the total impedance of all the speakers that are connected to it. Set the impedance selector in the back of your amp to match the load (speaker) impedance that you are using.
You will get the same performance (but not necessarily the same tone) with a 16 ohm cabinet by setting the selector to 16 ohms as you will if you use a 4 ohm cabinet and set the selector to 4 ohms. If the amp has no impedance selector (Fender Twin etc.) then choose a combination of speakers that comes close to the rated impedance... preferably lower.