It is good idea to have two separate letters.
Hello,
I did all my GC work (NIW) as a self petition: No lawyer and no company petition, just company support letter. Recently I received the physical GC too.
I wote one letter as petition which was about 15-16 pages signed by me. I made all exhibits containing first Petiton, Support letters, Awards, Publications, Present Visa copy, Passport copy etc.
Then I wrote a cover letter mentioning what kind of petition I am filing for and all enclosures (Petition with all exhibits). I made a transperent cover folder with Money Order and cover letter outside on the top. Then all enclosures with separaters and tabs classifying each exhibit.
This way it becomes very easier to INS officer to go through all paperwork in one shot. Also, if you are going for Consular Processing, your file goes to the respective consulate and there all your paperwork goes which they refer during your interview/scrutiny. That time too it looks good.
You may be planning to do similar way but I just wrote my experience.
Good Luck!