No, it's not possible to split up documentation like this.
The only place where it was possible to more or less do this was in case of some questionable use of record fields but does not apply here and we have changed this behaviour in 2.14.x.
Perhaps you could use a definition list to try and get some natural split in your documentation. For example, something like
class ModifMATH a where
-- | Explanations of simpMath method.
--
-- [MathDouble case] $part1
--
-- [MathMult case] $part2
--
-- [MathAdd case] $part3
simpMath :: a -> a
Please note that the docs refer to the 2.14.x version which is quite a a bit newer than what you're using so for example those newlines between each definition list are necessary.
It should look more or less like:
(source: fuuzetsu.co.uk)
You can go multi-line if needed, refer to the docs.