I have studied 120 credits at the same time (i.e. two modules running concurrently) and it was really, really difficult. I was also working full-time which didn't help matters! It is possible if you're very disciplined and dedicated. You do get your TMA deadlines in advance and can work at your own pace, so if you have more free time one week, you could use that to get ahead a little.
However as others have said, the results wouldn't be available in time for any September 2018 university intake which would be the main problem. Depending on how often the modules run, you could do one module in in Feb next year and one later one and then you'd have the results for both by September 2019.