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I don't really understand why you try to force a similar probability instead of creating an event with exactly the same probability. A roll of two dices is a sum of two numbers 1-6. Using 3 coins with values 1/2/4 you obtain numbers from 0 to 7. Ignoring the 0 and 7 (just repeat in those cases), you have exactly the same probability of a 1d6. Now do the same a second time for simulating the second dice and sum up the results.Said that, instead of using coins I think it makes more sense to have a dice roll app at your phone or, in the worst case, use a timer and the value of the last decimal as result for each dice (ignoring 0,7-9)