Fortunately, mountain biking and alpine skiing have a few things in common. In both activities, bursts of hard work (a climb on a bike; a downhill run on skis) are interspersed with more leisurely moments (flat stretches or downhills on a bike; sitting in a chairlift when skiing). So even allowing for the fact you’re probably skiing when temps are in the 20s, and mountain biking when they’re in the 40s, I think you can find a jacket that would stand up to both.
One thing I’d avoid is a very heavy, three-ply jacket made with Gore-Tex or some other waterproof-breathable, as I think it’ll be too much when you’re biking. Better are the “two-and-a-half” jackets, which combine a traditional two-ply laminate (waterproof membrane and face fabric) with a light inner coating. These are lighter, more flexible, and more breathable. Cloudveil’s Drizzle ($235; www.cloudveil.com) is one such jacket, incorporating Cloudveil’s proprietary Cloudburst Stretch fabric, long pit zips, and stretchy underarm panels. It’s cut pretty trimly, but I think would fit over most fleece jackets, except for the bulkiest ones. Patagonia’s Lightning ($210; www.patagonia.com) is similar in design and weight, but I know from personal use that while it’s a great jacket, it’s VERY snug, so layering is difficult.
I’d also highly recommend anything made with eVENT, a relative newcomer to the field. eVENT is a fabric that is chemically similar to Gore-Tex but engineered in such a way that water vapor can vent much more quickly. I have a Pearl Izumi bicycling piece (the Channel Jacket, $200; www.pearlizumi.com) that uses this stuff, and it’s borderline fantastic. Lowe Alpine is the latest company to use eVENT, so you might take a look at their Elite Jacket ($359; www.lowealpine.com), a nicely designed all-purpose shell that would be at home skiing, hiking, or on a mountain bike. The Channel Jacket is a good one, too, but I think it might prove difficult when you’re layering for cold winter days.
So there you go. Good luck!