Emtricitabine should not be taken with medicinal products containing lamivudine. There is no clinical experience as yet on the co-administration of cytidine analogues. Consequently, the use of emtricitabine in combination with lamivudine for the treatment of HIV infection cannot be recommended at this time.
Emtriva Summary of Product Characteristics, Gilead Sciences Ltd, April 2019.

Lamivudine should not be administered concomitantly with with other cytidine analogues, such as emtricitabine. 
Epivir Summary of Product Characteristics, ViiV Healthcare UK Ltd, February 2019.