We have two WCs that both share a waste pipe & transfer waste?

We bought our house new and have 2 WCs that share a waste pipe, one in the main bathroom and another the other side of a partition wall in an en suite. Trouble is flushing one can cause waste to be transferred to the bowl of the other. This discolours the water and if left for a few days can smell pretty bad..

How do we solve this problem? Can we get some sort of deflector or flap to fit into the waste pipe that directs the water down the pipe and stops the water transferring between the two pans?

Polk
July 2007