Not sure what you are trying to do, but you should almost certainly look at the environments provided by amsmath. multicol is really designed for cases where you don't want to specify the breaks but want TeX to figure out how to balance the columns without manual intervention. Where you have continuous text to set in multiple columns and want different numbers of columns on the same page, multicol is useful. But where you have
array material etc., it isn't really the best choice: it isn't needed and so you get all the downsides with none of the benefits. It's a lose-lose situation.
Possibly something like the following might be better: