Just ask yourself why you want to do a double degree. There's no need to like if you just enjoy engineering then dont do it. But if you feel your not getting enough maths or stats then continue it. But you're taking on a lot more work (and potentially debt), so make sure its worth it. As to employability, getting a job is really important, as that's the end goal of any degree. However don't necessarily jump on the whole, maths grads get jobs/stem proaganda etc. only take that stuff as a rough guideline. If you are passionate in what you study then that will flow on to other areas of your life and you will find something and make it work.
Just my opinion. Also my dad did mechanical engineering @ unsw (albeit a long time ago now loL) and he really enjoyed it but hated with a passion fluid dynamics and some acoustics thing. But apart from that it was enjoyable. He still has his slide rule and all of his drawings