Monumentous is a word made up of the words monumental and momentous.  While it is very common in the quasi-intellectual communities of the internet, it is not a real word.

To be monumental is to be great in importance, extent or size. i.e. like a monument.  It refers usually to things.

To be momentous is to be of great importance particularly to the future.  It refers usually to events, ideas, decisions, or changes.

They are similar but not the same and you certainly cannot make either one more powerful by adding the other.  Monumentous is a made up word that probably started as a mistake but carried enough memetic value by sounding impressive to propagate itself.  It is useless and wrong so stop saying it.