An uncle of mine once told me that a rabbi told him that having marital relations on Friday nights is a "double mitzvah," presumably because one would be fulfilling both the obligation to have pleasure (oneg shabbat) and the obligation to be fruitful and multiply (piryah verivryah = procreation). Where is the source for considering this a double mitzvah, doing two mitzvot b'vat achat?
תשמיש המטה מתענוגי שבת הוא, לפיכך עונת תלמידי חכמים הבריאים מליל שבת לליל שבת.
Marital relations are among the delights ("oneg") of Shabbat, and therefore the time for marital relations for a healthy Torah scholar is from Friday night to Friday night. (translation mine)
So the two mitzvot are Oneg Shabat and Onah. If you want to throw in Peru Urvu, all the better!