Here are three general guidelines to help you know whether you should be using one word, or two.
1. Use onto (one word) to mean “on top of,” or “upon.”
2. Use onto (one word) when you mean you are “informed about something,” or “fully aware of something.”
3. Use on to (two words) when on is part of the verb, such as “logging on to a computer.”
Onto vs. On to
