I know that the two should not be mixed, but i want to know that if the two become mixed in a contained unit, does it build pressure? Or can you seal the container safely untill it can be disposed of properly?
Yes, it will build pressure, and if you seal it, it becomes a potential bomb. Unless you're doing this in a controlled environment, intentionally, with a need for the toxic compounds you'll produce (chloramine, hydrochloric acid vapors, and/or hydrazine, all in a mixture of unreacted ammonia and bleach) you should not do this, period.
There is a reason that everyone is taught not to mix these compounds.