There are, though, a few potential pitfalls. Generally speaking, two bags will weigh more than one colder-temperature bag, due to the extra zippers and material that the two-bag setup entails. They'll also be bulkier than a single bag.
There's also the very real question of what temperature rating you end up with by combining two bags. Let's say, for instance, that you stuff a 20-degree bag into a ten-degree bag. What do you end up with? Impossible to say, really. You could calculate the fill weight of both bags and add that together. For example, if your 20-degree bag has 16 ounces of down fill, and the ten-degree bag 24 ounces, then that's equivalent to down bags in the minus 15 to 20 temperature range. That math should work the same for synthetic bags. ButByou have to factor in that the insulation of the inner bag will suffer some compression, so will not be as effective. So my guess is that adding a 20-degree and ten-degree bag will yield a bag with a rating of minus five to ten.
Hope that helps!