[joewatch]

This is a tricky one and I don't think there is 1 right answer. I think it depends on your table conditions and your reads.

I do think that a 13-out str8 draw and 17-out str8 draw are worth it even when there is a 2-flush on the board, but only if it's head-up and you have position. Without position, you don't have the correct implied odds, especially against a tight villain. If it's not head-up, you might be splitting with somebody with the same or better draw.