There's no real problem, per se, with stacking filters, but you'd want to try it first when you're in a position that lets you test things out, not do things and later regret. Main things to look for are loss of sharpness, skewed colours (heavy colour cast that you can't seem to fix without murdering the image), and flare (try shooting toward the sun, and at different angles). Take some test shots, first without the filter, then with one filter, then with the other filter, and then with both stacked. Also try flipping between them—put one filter over the other, then take both off and the one you put second before, place it first this time. It shouldn't really make a difference, but I guess it's worth trying.