It is manageable, but not by many people. It's pretty hard for us to know if you're good enough to do it. But, if you really want to, you could try it at AS level and drop as required.
There is very little point, though, in terms of admissions. You can protest all you like, but it's just true. Oxbridge tutors want to see high percentages in your most relevant or best subjects, as Protoxylic correctly states. Beyond that, it's all down to admissions tests, submitted work, and the interviews.
I think you're a troll anyway, but this is the advice I would give to someone genuinely asking the question: assuming you're capable of handling the workload, take the extra subjects if you want to do it just because of interest in the subject, but not if you're doing it for admissions.