CA strongly recommends against installing and running 2 agents simultaneously. It is not documented, has never been tested and QA'd and as such is not supported. Even when the installation succeeds, you may run against unforeseen problems later on when both agents are running.
Keep in mind that, during an upgrade process, it is perfectly alright to have a lower release agent report against an already upgraded execution server. Perhaps this eliminates the need to run agents side by side.