By on the turn you mean on the flop right, since the odds of hitting two particular cards with one card left to peel is zilch. For all intensive purposes (yes I know it's intents and purposes guys... derp) a backdoor flush or straight can be counted as approximately 1.5 outs. A runner runner flush comes in 10/47 × 9/46 or ~4.16% of the time. A runner runner straight with the most likely cards to do it with (i.e. 3 consecutive ranks all > 2 and < K) comes in 16/47 × [(8+4)/2]/46 or ~4.44% of the time. Of course, translating these figures into an exact number of "outs" is going to depend on the total number of outs you have. Also note that in practice the effect on your actual
equity will be lower since your opponent also backdoors a stronger hand sometimes or is going to have redraws at the very least.