Is it possible to run multiple versions of MySQL Workbench?
I don't see why not. Are you encountering errors installing two versions at once if you specify different install paths? You may have to use different versions of the MySQL installer as well.

It does look like Workbench 8 is compatible MySQL 5.5, though (and I can confirm because I have been using it). The documentation says some queries will not work like showing the processlist (although I just tested this and I can SHOW FULL PROCESSLIST of my MySQL 5.5 server with MySQL workbench 8.0.12.)