Load sharing between two ISP's

Hi,

Router: Mikrotik CRS 125,
we are iselling internet in india, we have two uplinks, now my concept is i want to share internet bandwidth.
i.e: we have
Up-Link-1 : 100Mbps
Up-Link-2 : 50Mbps

if Up-Link-2 bandwidth reaches to maximum (50Mbps) traffic has to be pass through Up-Link-1 and vice versa.

Review this for what you need to accomplish..

http://mum.mikrotik.com/presentations/US12/tomas.pdf