I've done As and A2 Further Maths in one year (final year, 3rd year, 2014/2015). I loved maths, but around the time I started 6th form I was scared that it might be too hard. To be noted: by the end of 2nd year I achieved A in A-level maths. At the end of 3rd year I retook A-Level Maths and achieved A*, in A-Level Further Maths I got A, and in A-Level Computing (3rd subject) I got A. In the end I only had 3 A-Levels, since I kind of messed up in Secondary School and As Physics in College. There was also some one in my class doing same subjects, but was doing A2 Maths (,unlike me) along side A-Level (AS and A2) Further Maths. He got an A* in Further Maths, because he studied properly (obviously, I regret for not getting the A* like I promised to my teachers). For the record with both did STEP (we both got a 2 in it) maths and AEA maths (I failed somehow and he got a Merit).
Further Maths modules in my College: Statistics 2 (S2), Mechanics 1 (M1), Mechanics 2 (M2), Further Maths 1 (FP1), Further Maths 2 (FP2) and Further Maths 3 (FP3). I did Statistics 1 (S1) for A-Level Maths.
For Further Maths, I had to learn FP1 (a module, I guess) before the 3rd year started, otherwise I would had difficult with modules in A2 Further Maths (FP2 and FP3). Over the summer holiday I watched all Exam Solutions' Videos. I should have also read the FP1 book and done all the exercises in it, though instead I did them just before the lessons over the year. Over all I found FP1 easy, and it didn't interfere with other modules.
For A-Level Maths you will have to learn C1 (Core 1) and C2 (Core 2), though they are changing the exams, if your exam board is edexcel exam (like mine). C1 is rather easy and mostly reminder of Secondary School Maths. I personally was good with Maths so I would say I found A-Level maths easy and would highly recommend doing Maths in one year. But I also would consider if the current A-Levels I have is enough to get into the University you want. There is no reason to overload your self. Depression won't kill you most likely, so it should make you stronger, but there is the possibility. As far as I know Economics does not need integration and geometry (and etc), so stats would be better. Though I found core maths easier.
KAKAKAkA!