<p>I'm using ReportViewer WinForms, and since it is no easy way to create an coversheet, then I wonder, is it possible to render two reports and have them concatenated?, so they appear as one report?</p>

<p>If I was to print only, then I could execute two reports after each other, but since the user want to see the report before printing (you know, no environment waste here) then they have to appear in the same viewer.</p>

<p>OR, is there other ways of creating coversheets?
Today I use an subreport, but there are some issues with margins etc. which is not easy to fix.</p>

<blockquote>
  <p>To clarify, we are talking about
  ReportViewer using RDLC files, no
  Crystal Reports involved.</p>
</blockquote>

I've created a report that sounds like what you are attempting to do...first to clarify, I'm going to guess your using Crystal Reports within VS2005/2008.
If that's the case, all you need to do in the main report is create an additional section after your section that contains the "Cover Sheet" layout/data. In the section expert for the "Cover Sheet" section (in layout view, right click on section header bar, pick section expert in pop up menu..), check off the "New Page After" option.
Edit: After your update, I see you are using RDLC reports, and from my limited exposure to those, I can't recall an easy way to get to where you want to be. Though I'm pretty sure you may be able to pass multiple reports to the same report viewer in code.