I completed Psychology 3/4 last year and to be honest I found it a little more relaxed in Units 1/2 compared to Economics 1/2 (which I also tried and dropped last year; it wasn't really my thing). Economics does require more effort and for me, missing out on days meant that I was pushed for time a lot. There's a lot of content to be learnt and I think you'd need to be very consistent in learning definitions for it (Psychology is similar, but to a lesser extent, in my opinion). Depending on the sort of person you are, you may find it to be enjoyable.
Psychology 1/2 was relatively okay, but of course, put some effort and time in to revise and you can push a 90% just fine. Do it like you would to any subject.
Overall, it was just what you made of it and with a little luck (and time) you should be fine. I didn't really find that my Economics work interfered with my Psychology work, because the SACs were more often than not in Psych than Economics, but I didn't do both together at the same level, so your experience may differ significantly.