A 5.2 person can potentially do 2 unit, although they will need to catch up on 5.3 content because it is assumed knowledge in 2 unit. Most people get streamed into Year 9/10 maths classes based on their performance in Year 7 and 8. So often times a few people who are in 5.2 have the potential to do a higher level of maths or have improved significantly over Years 9 and 10. However, I would only consider 2 unit if you are doing very well in 5.2. If you are an average 5.2 student or if you are struggling, then 2 unit is probably not for you because it is a big jump between the maths levels.
In terms of a 5.2 student doing MX1, then I would still say it is possible for the same reasons, although very few students in 5.2 would have the potential and many would fall into the category where they need to work extremely hard to succeed at that level.