It somehow reminded me of the preposition "onto", so I thought in this special case it wouldn´t be a lunacy to say that "on to" could be a specific case of "onto".
That said, onto
is a preposition used to talk about something in a not fixed position (it should be used on
in that case). So, onto
is used to mean something that moves itself
OR something that moves from a location to another
. Example: transfer the data onto a disk.
Then I had it confirmed...here:https://en.oxforddictionaries.com/usage/onto-or-on-to
In Portuguese Ricardo has nailed it, the translation is spot on!