This response is based on the information provided.  We suggest you discuss with your contracting team, program manager and/or legal department as appropriate. 

We think you are asking if you can limit the Fair Opportunity to other than the two companies that are winning most of the IDIQ orders.  We believe you need to justify excluding two of the eight companies from submitting simply on the basis of "those companies always win".  FAR 16.505(b) does not offer a clear process to exclude offerors within the framework of data you provide here.  There is a reason those companies win.

Can you do a set-aside for some opportunities that would preclude these companies from submitting an offer?  If not, i.e., one or both is a small business, you can look at the selection criteria you use to evaluate.  Based on your submission, these companies are struggling to meet schedule on awarded projects.  Schedule and past performance could be part of your streamlined evaluation criteria.

Another possibility is Full and Open Competition After Exclusion of Sources.  That will require approval of Head of Agency or Designee.  See AFFARS 5316.505, AFFARS subpart 5306.2 and AFFARS 5306.304(a).