Should I contact the reviewers I suggested during the submission if the manuscript is still under review after one year? In order to publish my paper at The Journal the cover letter should include four (4) experts in the field as potential reviewers for the manuscript. I ask some professors to have their permission to include their name as reviewers and find 4. I sent their details to the Journal. I didn’t hear anything back from the journal after 8 months, then I sent an email asking about the status of my manuscript and they indicated that they received one review for this time and they are waiting for the second. Now almost one year has passed for my manuscript, and the Journal is still waiting for the second review. 

Should I send an email to the reviewers to ask them to read my manuscript?
No.

You don't (and shouldn't) know who your reviewers are.  The editor could have ignored your suggestions entirely, or even used them as a list of conflicted people not to ask to review your paper.

Feel free to bug the editor again, but leave it to the editor to bug the referee(s).