I think we are misleading students when we tell them the "rule" that two-syllable adjectives form comparatives with '..er'.
In fact, it's mainly the two-syllable adjectives ending 'y' which behave in this way : busier, prettier, noisier, friendlier and so on.
Most of the others actually follow the 'more'/'most' pattern. There are several categories of adjective which have to do this - for example, adjectives with certain suffixes ( helpful, useless) and adjectives that are verb participles (boring, bored).
There are also a few where both forms exist : for example, you can say 'cleverer' or 'more clever'. So, if in doubt, you should always use the 'more'/'most' pattern - it's far more likely to be right. While 'opener' or 'boringer' are definitely wrong, you could easily get away with saying 'more busy' or 'more happy'. The 'more'/'most' pattern is generally a much safer option for all adjectives.