These runner runner outs (where you have to hit two cards) are seldom worth a full out. Note he could also hit 10 and 4 and 9 and 4 for a weirdly better two pair.

In the case of backdoor flush draws you can give a full out for both cards hitting because their are 9 and 8 cards to choose from. Here you have 6 cards and then 5, so it's worth maybe half an out. Usually, you can just ignore these back door outs in your calculations (not really true in limit hold'em).