Hi,
I just got last week a vintage (year 2000) Ampeg SVT3 Pro, with 2 "classic" series cabs, a SVT-210HEN (2 x 10") and a SVT-15EN (obviously 1 x 15" cab).
I have connected to the "speaker output" ports the jack cables, normal 1/4″ size, one to one cab input jack, the other to the second cab...