You can get the merged coverage information for all your runs using the CoverageInfo class.
string ccFileMerged = Path.Combine(Path.GetTempFileName + ".coverage");
CoverageInfo.MergeCoverageFiles("A.coverage", "B.coverage", ccFileMerged);
// now you can use the ccFileMerged to get your merged data
However you should note that the merge will group dlls based on some static information eg. dll guid, which is regenerated per build. So as long as you ensure that all the .coverage files were generated with runs consuming the same dlls the merge will be seamless. If these dlls are generated per run, you will end up seeing a dll entry per unique dll in the .coverage file