But I'd be concerned that if you are already doing full time (120 credits) then to do additional credits on that would be hard going...not impossible but you really need to think about it. I did biology & chemistry at a Durham uni and that was pretty full on couldn't imagine doing more on top of it (turns out I wasn't cut out for full time studying...but that's another story
) . Although I did get to take some Maths modules as part of it. Then completed a different degree at the OU (Maths & Computing).
Not only the time and effort needed but the cost is higher these days for a module with the OU. (Thanks government!) It's around the £1300 mark for a 30pointer and around £2600 for 60 pointer. So any extra modules will have to come out of your own pocket if you already have a student loan for current course.
The OU only allows you to do 150 credits in exceptional circumstances but not sure how it would work if the credits are done at another uni. Best to call and ask and at least get some brochures sent out and have a look over the website. But I personally think you might be over doing it and it might negatively affect grades on original course not to mention eat into spare time. But these are things you will have to weigh up.
A general rule is 9-10hrs for a 30 point module a week and around 20 for a 60point module but it varies between modules and how well you take to it. But I can imagine it might be horrendous if exams/assignments end up being due at similar times though.