In my opinion, it doesn't mean much by itself.  It can mean double the amount, or it can mean by two separate processes.  It could even mean by the same percentage twice, which would be different from double the amount.

If it means double the amount, we have to ask "what amount?"   If it means two separate processes, we have to ask "what was the first process?"

It's tempting to say that it cuts the concentration in half, but I don't think this is at all justified.

Most likely, previous context would reveal a process for reducing the concentration of these compounds which is entirely different from the thermal treatment process.  By using both, there would be a greater reduction in concentration than if either one were used alone.  But there's nothing to say the reduction would be twice as much.  But it could be described as twofold.

Another interpretation would be that process A reduces the concentration by 20%, but if you use "thermal treatment" instead of process A you can reduce the concentration by 40%, that is, by twice the amount.

I think "while" is definitely the wrong conjunction.  The two clauses bear very little relation to one another, and probably should not be connected.  I'd suggest two separate sentences, if not two separate paragraphs.

  - A.