In the last job I had, I argued the stupidity of having two BT lines for redundancy.
You have two providers, two paths in and a spare router so, barring a power cut, you're fine and as with anything like this you have to know how far to take things.
If you can also add in a 3G/4G card then you've gone as far as can be called reasonable to ensure your business continuity.
I don't know enough about the Draytek to say whether it does or doesn't, but check that it can be configured for load balancing and resilience if one goes down. Many routers I've seen, for some reason, are configured for one <b>or</b> the other and I see that the site says "The multiple WAN interfaces can be used either for WAN-Backup or load balancing"
As a tight yorkshireman, having one line sitting around unused makes me shudder - if it's not automatic switching between the two then I'd recommend you configure for load balancing and manually reconfigure if a line goes down (I'm assuming instant switching isn't critical?).
But I've heard good things about the draytek, so hopefully it's plug and forget.