My experience having been a bicycle racer, triathlete, and runner all separately is that there is a definite cross training effect when you train both sports at the same time. I can run more or less equally well on 45-55mpw and 5-6hrs of relatively intense cycling as I can ~75mpw running. I guess that would indicate, for me, 1hr of running has about the same training effect as 2hrs of cycling. It obviously doesn't work to the limit, however. If I switched to cycling exclusively, I would obviously get much slower at running regardless of how much I rode. It is difficult to know where the sweet spot is.