The reason is, I am familiar with 5.5 and can used for my daily work until I get used to Trados 6.5.
That won't be necessary, as the "look and feel" is the same.
Is it possible or advisable to have both versions of the program on the same computer?
It's definitely possible. What you need to make sure is that you register each version before using it. "Registering" means making certain settings in your computer's registry: in T6.5, there's an entry under Start - Programs - Trados 6.5 - Register Trados 6.5. In T5.5 you need to look in the TT directory of the Trados installation folder: there's a file ending "selfregister.bat" which you need to execute in order to prepare your system for T5.5.
A few additional comments:
The latest version 6.5 is designed to supersede the previous one.
Correct, but T5.5 will not be deinstalled automatically. You can deinstall it before installing T6.5, but you don't have to. In fact, if you wish to continue using MultiTerm 5.5, it's probably better to keep T5.5 installed.
I use the same configuration. In particular because import of glossaries from tables to MultiTerm formate simpler in version 5.5.
You can use MultiTerm 5.5 with Workbench 6.5 - just change the Terminology Provider settings in Options - Term Recognition Options in Workbench.
Having said that, I find it much easier to convert Excel tables (for example) in MultiTerm iX - have you tried the MT Convert utility?
How can they both function without interference in their respective smooth functioning?
See above - registering each version will ensure there's no interference; you can never use them concurrently, though.
Best regards, Ralf