if they're both 8Ω then there's less chance of a mismatch.
Two 8Ω cabs in parallel will give you a 4Ω load and in series will give you 16Ω.
The danger comes when you present a load (ohms) that is too small for an amp, some valve amps will tolerate a 2Ω load, but most amps state 4Ω minimum.
The minimum impedance (ohmage) should be on the back of the amp near the speaker connectors.
So long as you don't give it more than about 300W then you should be fine .
Nice looking rig, by the way.