There are two of us also. We installed a new 1000 gallon concrete tank about 5 years ago. (The old one was a site built tank installed in 1948 and it caved in.)
Two years after we installed it we pumped it out and the pumper said, based on what he saw and there just being two of us we could likely go 4 years between pumping. As an aside we also put in a new drain field that uses shallow diffusers and it has performed well.
