i'd redose after you come up or like 6 hours in if you want an extended trip. IME if you want the effects of 25mg for 18 hours, then you have to redose again with 25mg. If you are just trying to avoid the nasty come up of 25mg, then take 15mg, come up and when you feel okay, take the other 10mg and you won't get another come up or have the anxiety/nausea of a come up. However it won't be as strong as 25mg at once. If you're looking for that strength then dose 15mg and 15mg 2-3 hours later.
i've redosed 2c-e so many times but i think it comes down to preference and what you are trying to achieve by redosing.
i wouldn't redose until 2 hours in at least, as 2c-e takes quite a while to get going.