Pure maths is hard. So is maths and actuarial science. That's a lot of stochastic calculus. Doing a joint degree does put the pressure off. The most that economists tend to do here mathematically is differentiation, and doing a joint degree does help you see what all that theory and algebra that you do in maths is about.
If you enjoy maths and have a passion for it, do it. But if you want to do a combined honours in two subjects you enjoy, there are some serious benefits (like the fact that you don't have to do maths you hate).
What if you end up doing maths and actsci and you end up hating stochastic calculus? Same thing with a pure maths degree, really. Many people took it and realised they hated doing so much maths at once. I'm one of the lucky few who enjoyed it all.