Well, no I did not dive with 2 SPGS before AI, any more than I would expect two SPGs or a SPG and a wrist computer on most rental gear today. In fact, when I started diving in 1965, I was cave diving with no SPG or computer because there was no such gear. We just had a single Voit Avalon regulator, a plastic backpack with a 72 cf steel tank with a J Valve, homemade full 1/4" wetsuit and homemade weights, USD mask, snorkel and fins. Did that mean I felt safe, or would not have had a serious problem if something happened to my regulator or air supply? Of course not. Today, we have the availability of all the safety and redundant dive equipment you can ask/pay for. Fortunately, I can afford to dive with as many computers as I like, or an AI and a SPG, or any combination of gear that I might choose. If my AI or SPG fails, I finish the dive. If both fail, I end the dive, and change out whatever failed during the SI. Since I don't dive as much as I would like, when I do get on the boat, I want to make the dives, and do whatever I can to insure that happens.