Trying to control massive magical energy to dissipate an enemy spell is extremely taxing and difficult, and can easily go wrong. Therefore, just as with casting spells, if two or more of the dispel dice come up as 1s, the attempt to dispel a spell automatically fails. Regardless of how many bonuses you might have accrued from magic items, etc, a roll of two or more 1s is always a failure.
If a player rolls two or more 6s when dispelling a spell, the dispel succeeds automatically. Note that an automatic fail takes precedence over an automatic success when dispelling a spell. For example, the player rolls five dice for a dispel attempt and gets two 1s, a 4 and two 6s. Even if the total of the dice is enough to beat the casting result and the player has rolled two 6s, the two 1s take precedence, and the spell is cast successfully.