Having two outputs that say "4 Ohms" does mean, that you can plug in an 8-Ohm - cab in both of them, and because the outputs are wired in parallel, you will end with 4 Ohm total.
So you can use the 4-Ohm - jack on the back of the Ampeg-cab to do it right. No need for use two cables.
I had a Randall-head like this back in 1979, was a fine sounding amp theese days. Congratulation for this gem.
So sayed Kong.
Official Mark Bass Club #348 / Genz Benz Club #360 / Official Fender Precision Bass Club # 1029 / Official Ampeg Club # 893