The issue isn't getting them all to work together as in VT they are all doing on-demand scans on a single file and you can see how long that takes.
The main issue when talking about multiple AVs is that the resident ones to be able to do their job have drivers that are running to intercept (hook) calls to run a file, so it is first scanned before being allowed to run. It is these drivers that cause most of the conflict issues. So VT isn't the same as having multiple AVs installed.
So there are times when because VT is only on-demand that they don't replicate real world detections, where the other shields and scans may be able to detect malware were VT doesn't.
Do you really think that all of these paid programs are going to allow their databases and engines to be used in a co-op of virus scanners. I don't think so unless they are paid.
There are a few examples of AVs with two AV engines and virus signatures, but only one interface controlling the hooking (so no multiple drivers running to conflict). But with exception they are paid options because the company has to pay a license fee to use the intellectual property of the other scanners.
I certainly wouldn't recommend hitman pro as there are many instances of an overly aggressive deletion of what may be legit files.