Hi Karen- that looks like a double. Two visible stems fused together. I would let a few more buds develop to see if you get a megabloom. I have had as many as 5 or 6 fused with Big Zacs in the past, and yes, the cold nights seem to produce the most multiples.
Many times, only one or two get pollinated in a fused stigma situation, and a badly lopsided tomato results. I have the best luck by taking another flower from the plant, and smearing the entire stigma of the megabloom with the pollen from the sacrificial flower. You'll want all of the ovaries on the megabloom to become active. It usually helps me
