The Australian Oxford Dictionary defines two-up (or swy) as "a gambling game in which coins are spun in the air and bets placed on a showing of two heads or two tails".

Two-up at Adelaide River, Northern Territory Adelaide River, Northern Territory, 1943. Two-up games conducted to raise money for the prisoner-of-war fund.
AWM 057097
Two-up at Ypres 1917 Ypres, 1917. Australian soldiers playing two-up.
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