Is there a way of having 2 links terminated to a router OS, so that when one goes down it automatically picks from the second provider.

Look for bonding interfaces or ospf, it depends on what you need to do.. Perhaps with static routes its possible too. Need more info. Adsl link, wired? Wireless?

Here is the scenario.
I have a Wimax from my provider which is terminated to my RouterOS via cable on ethernet1.
My capacity is 512kb up and down.
I then have my hotspot on ethernet2 which my users are connecting to for browsing the internet.
The problem is when the link is down then my customers have to suffer.
I want to take a connection from a different provider as well, which again will be cabled and terminated to ethernet3 to act as a backup link.
Since it is going to be rare for both links to go down at the same time, customers will enjoy better uptime.
How can I configure both the links to work togather....

in the simpliest case you should just add two default routes with different distance

also you may need to use something like http://wiki.mikrotik.com/wiki/Advanced_ ... _Scripting