Multiple applications accessing SQL Server
It depends on a few things.


SQL version. Will the new application run on the same version of SQL as the existing application?
Security. Are you comfortable having the same people possess server-wide permissions to both applications?
Performance. Are you comfortable with a runaway query in one application possibly affecting the performance of the other application?
Maintenance. Are you comfortable with the idea of any maintenance that requires a server reboot or a SQL service restart taking both applications out of commission at the same time?
Vendor support. Do the vendors of each application support having them co-located on a server with other applications? (Just because the vendor doesn't like it, doesn't mean you can't make it work. But if you don't follow the vendor's requirements, you run the risk of them refusing to help you when something goes wrong.)