You can always get a copy of mstsc.exe off a computer that has the old version and just drop it in the windows folder. Or if you don't want to screw with the company's windows folder(or you don't have access) you can create your own using a MSTSC from another computer.
Start out by collecting the files from a computer with RDP 6.1 (your old version).
Files you will need:
Note: en-US is for the english US version if you are in the UK it will be en-UK France is fr-FR you should be able to figure out which folder is your default language and find the mui's based on that.
Now that you have the files create a folder (wherever you want, this can also be done on a thumb drive) called RDP (or a name you like) then drop mstsc.exe and mstscax.dll in it. Next create a folder inside your previous folder (RDP in my case). The name (case sensitive) and location of this folder matter it must be named en-US (or your language code as discussed above). Drop the two MUI files in that folder, launch mstsc.exe, and you now have two versions of Microsoft Terminals Services Client on one PC: