It depends on what your goals are…

Do you want to work an academic career? As in, PhD, post-doc, continued research?
Do you want to get out of SA for a while, or even live outside SA thereafter?

Doing the second masters wouldn't hurt your opportunities of funding a PhD afterwards at all (unless you flunk it). It may even increase them, with you having experience of studying abroad, it would suggest you're able to work a PhD abroad.

It will also depend on your circumstances…

Do you have kids/think you might have them in the next few years?
Do you think you'd still want to do a PhD when you're 29?

Personally I'd say do the second masters. It won't hurt your potential academic career whatever (if you do well on the course), and you'd be studying in one of the greatest countries there is – seriously, they do most things right over there. It's one of the most difficult countries to 'get in to', and you'd sort of have your foot in the door. Go for it!

I'm at a critical-decision point in life/career and I would really like some advice.

I want to teach and be involved in education, but I'd like to do this without the research. Indeed, I would like to do this part-time, allowing me to study independently and maintain other interests. Salary isn't so much of an issue.

I understand that a natural suggestion would be school teaching, which is a good career with good holidays. However, I feel that I would get bored teaching lower-level subject (it would be physics or maths or chemistry in my case) for many years, and would also not enjoy classroom-management side.

Another route I'd looked at is Associate Lecturer roles with the OU, which would let me study and teach at the same time. However, I've been put off by many negative reviews regarding workload/pay balance and negative structural change within the organisation (which many feel is destroying the OU).

I have a first class Masters in Chemical Engineering and further masters degree in membrane engineering. I'm studying standalone applied maths modules with the Open University.

If anyone can share anything at all that could help, I'd be very thankful.