Hi,
It is relatively difficult to answer this question without more information. First, the best thing is to use the same speed rated, size and width tires on all four wheels. Also, the tires should have the same tread pattern (this will optimize handling, especially in bad weather). BMW recommends that all four tires be of the same speed rating, size, height and width. They also recommend using a full set of M+S winter tires to prevent problems with ABS braking in the winter.
You might be able to get along with two sets of different tires, two in front, two in back, if the are the same speed rating, size, height and width. The problem is that your handling and braking will most likely suffer (due to the different tread patterns) and you would have to exercise great caution until you got the "feel" of the vehicle under these new conditions. If you are just driving around town at low speeds and not driving at high speeds on the highways, you could probabvly get away with this.
Still, if you had a choice and the money, it is best to keep all four tires on your BMW the same make, tread pattern size and width. This is my opinion only, and other BMW drivers might completely object to even thinking about using different tires on the vehicle.
Hope this helps.