Hi,
Actually you don't need a separate VI client to access vCenter server 5.0 and vCenter server 5.5. From my understanding, I believe you have already installed VI client for 5.5.
Just try to access the vCenter server 5.0 from the VI client which you installed and it will prompt to either Save or Run the required binaries. You can either save it and install later OR directly install by opting RUN.
Once the binaries are installed, you can able to access both vCenters from same VI client.