onto Versus on to

The preposition onto written as one word (instead of on to) is recorded from the early 18th century and has been widely used ever since, but is still not wholly accepted as part of standard British English (unlike into, for example).

Many style guides still advise writing it as two words, and that is the practice followed in Oxford Dictionary.

However, onto is more or less the standard form in US English and in the specialized mathematics sense.

Nevertheless, it is important to maintain a distinction between the preposition onto or on to and the use of the adverb on followed by the preposition to:

She climbed on to (or onto) the roof but let’s go on to (not onto) the next point.

Onto  is now generally accepted as a word in its own right. On to  is still used, however, where on  is considered to be a part of the verb(adverb):

He moved on to a different town

as contrasted with 

He jumped onto the stage

On to: Meanings:
:moving to a location on the surface of:
They went up on to the ridge.
:moving aboard (a public service vehicle) with the intention of travelling in it:
We got on to the train.

:aware of the true nature, motive, or meaning of:

I'm on to your little game.

The Chicago Manual of Style offers this trick for determining whether "onto" or "on to" should be used. Mentally say "up" before "on". If the sentence still makes sense, then "onto" is probably the right choice.

There is no difference between them. both of them means upon & to. the difference might be int he writng and colloquial positions.