Can I make a crossover or buy one?

My son got some small B&W speakers for Christmas and their low end bottoms out around 40-50hz, and if my guess is right it's probably down at least 3db at that range. So I bought him a passive sub, which is nothing more than a 12" woofer in a box. What I need now is a crossover that both combines L&R channels for 100hz and below(or there abouts), but also provides a high-pass for 100hz and above for the B&W's. If I can just buy a unit that will do all that I would probably just fork out the bucks but I can't find one. Twenty years ago or so I went to an electronic trade school (while trying to find myself), but have forgotten just about all I learned. But if someone had a schematic I may be willing to try. So if you know of a brand/model that will accomplish this or have a design could you let me know?Thanks, Jim