That depends on how the jacks on the cabs are wired. Each cab is 8 ohms, and I presume they have 2 jacks each? They're most likely wired in parallel, which means the 2 cabs would result in 4 ohms.
Wire them together, disconnect from the head and measure resistance across the pin and shell of the jack that normally goes to the head. Expect to see 3-5 ohms. That would mean a nominal 4 ohm load.