|I have to configure dual gateways in system and add the routes for MPLS but this scenario doesn't work well.|
What exactly are you trying to do this on? A Windows based PC? A UNIX/Linux based PC? A network appliance??
You need to be connecting the two external connections to a dual WAN router of some sort. Preferably one that either does load balancing or else automatic failover depending on your needs.
I should be able to access both networks on single Gateway IP without adding any routes.
A dual WAN router will accomplish this for you as the router itself will handle the two separate networks and their associated gateways. From the LAN side of things, you'll only have to worry about having a single gateway.
It matters not how straight the gate,
How charged with punishments the scroll,
I am the master of my fate;
I am the captain of my soul.