( polyethylene glycol
) is a laxative. there is no harm if you have taken two capsules of the same. They should have advised you to take the capsule at bed time as it would be convenient for you to pass stools in the morning without any discomfort. You have already taken 2 capsules there is no issues. The reason why they have asked to drink more water is becoz the drug acts on the fecal mass by osmotically drawing water into the GI tract and softening it to an extent.
Minor side effects like nausea
, gas and bloating
might be noted but they will go off as the action of the drug wears off..