If you take engineering, you major in engineering and not maths.
Generally, engineering graduates will be accepted by MOE to teach
Maths or Physics or even Chemistry if you do chemical engineering.
While you're doing undergraduate studies in Engineering, you can
take elective Maths modules (but limited to those offered for
cross-faculty by the Maths department) on top of your core
Engineering Maths modules. This will show up in your transcript
(i.e. the list that shows all the modules you've taken) but not on
your degree scroll. A minor is when your university has that
particular scheme that says if you pass enough modules from another
faculty (e.g. you are in engineering faculty but you take enough
elective modules from business faculty, you may qualify for a
business minor on top of your engineering major) and it shows up on
your degree scroll if I recall correctly. In my time, there was no
Maths minor for engineering students in NUS, and I doubt there will
ever be locally. Not sure about overseas universities though.
If you're very sure that you want to go into teaching Maths
only, you might probably be better off doing a Maths major, because
most of the stuff you have to learn for an engineering degree
(maybe 95%) will be irrelevant. Unless, of course, you like to
study just for interest and to challenge yourself.
Opinion coming from an engineering graduate who is finding her
Maths rusty 
Not sure about NUS but NTU does offer a minor in maths now since
i've friends who are doing it now.