Send them together. Also, put the RFE letter copy on the top. That's all.

I did the following: my RFE was only for medical exams and in each sealed envelope (that the civil surgeon gave us) I placed the copy of the RFE and sent everything (my family and I) together in the same envelope as priority shipping. I did not send a cover letter, I already have the update where it says that USCIS received my response.

thank you @C R @Charlie SC ! my lawyers taking forever to give me a cover letter, so glad to know your experience

@Sunny B form must' be filled by surgeon and given to you in a sealed envelope. send that sealed envelope together with RFE letter , by or before due date. all family members medical should be together.

was the RFE sent on medical before the biometric request? or you already had biometric done?

@T S I had sent Medicals with the initial package. then we had biometrics. after that we had 1st RFE for birth certificates then a second RFE last month for medicals