we can do something but it is not real clustering .
As i said it is not real load balancing . load balancing should be transparent .
You can do something to this effect by using identical hardware to create two or more mikrotik routers. If they are both running a PPPoE server in the same broadcast domain, the client will connect to the first PPPoE Access Concentrator which responds. With machines running on the same hardware with the same config, connections should balance themselves between your mikrotiks.
You will need a separate pool per mikrotik, or let radius assign the IPs and run a dynamic routing protocol (like OSPF) between the mikrotiks.
Assume user1 connected to router1 and user2 connected to router2 . if router1 fails user1 will be disconnected and should again connect. It is just a simple example