With many two-syllable adjectives (e.g. polite, common) -er/-est and more/most are both possible. With others (including adjectives ending in -ing, -ed, -ful, and -less), only more/most is possible. In general, the structure with more/most is becoming more common. To find out the normal comparative and superlative for a particular two-syllable adjective, check in a good dictionary.
This is the website I have shown in class