Is on to always one word?

Answer 1

The preposition "onto" means placed on or atop something, as in "he lept onto the roof" or "photos had been glued onto the wall."

The different pairing "on to" is a rarely-used form where the adverb "on" means moving or heading toward a destination, as in "we will stay here while you go on."

The use of "on to" is to indicate a final destination -- he led his troops on to Rome means he led his troops on, to Rome. This is often seen in military histories as "he led his forces on to victory" where on is not actually needed to convey the same meaning.

Answer 2

One word.