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The risk of loss is not the same. It's nearly doubled in the scenario where one keeps the same amount of data on one card vs. two.
To understand this, you have to first understand that the risk of failure is very low in the first place. If a card fails, it's almost certain that the second card will not fail on the same outing. Thus in almost all the scenarios where a card fails, the two-card approach loses only on (approximate) average half of the data.
In addition, though there are times, so rare it'd be like winning the lottery, that both cards would fail concurrently, they are much rarer than when the single big card would fail and are essentially negligible. But even in that case, one's no worse off than with the single card.