It sounds like one of those teeth has a fractured cusp. I am not a believer of replacing amalgam fillings unless it is absolutely necessary and if it has to be replaced it does not have to be replaced with composite restorations.That being said it is possible that the tooth has been weakened by the extent of the fillings that have been present. I would certainly consider seeing your dentist. If you can not bite on things that are hard something is not right in Denmark.
your case is precisely why i do not remove old amalgams unless I see evidence of decay or fracture (either in the tooth or the restoration). Simply dilling on a tooth can cause small fracture lines in the tooth structure (this can be minimized by using diamond, as opposed to carbide burs) which can become bigger. Simple drilling can also irrite the nerve. And you are right about the wedge effect, but now, since the "hole" had to be made bigger, you even more of a wedge effect. Pain upon biting is a sign of a fracture, although the pain can be for other reasons like sensitivity to the materials used in whirte restorations, or an incomplete curing of the materials. The fact that you can see a crack line may be important. If your current dentist cannot offer a solution, you may want to get another opinion. Also, you should not be getting food caught anywhere--- this is a sign that the restoration(s) were not contoured/trimmed correctly after they were placed. Teeth with very large fillings--- white, silver, blue, pink or magenta should generally have crowns placed. this will eliminate the wedging effect by covering the whole tooth and directing chewing forces down the long axis (roots) of the teeth like nature intended.