Completely depends on the servers.
Some servers aren't affected by it at all while others may stop working.
For example:
If both servers reads the "data.ini". They would both replace that file with their own and the other wouldn't work unless you change it back when you want to play the other server.
Or if both servers are using a data folder. But like Pow said, you'll rarely run into servers like that.. Most servers have their own GRF file these days.
Another problem would be if the servers you want to install in the folder both have similar names. Like "ExampleRO" and "ExRO", both of them could name their GRF file to "edata.grf". Like the other examples, 1 of them would replace the other.
That's just some examples on what could cause trouble by installing 2 servers in the same folder. But, a lot of servers out there have their own "data.ini" with a different name and are using GRF files. So normally you wouldn't have any problem with installing 2 or more servers in your RO folder.