The Cowboys had one roster spot available but released wide receiver Lavasier Tuinei, who was on the non-football injury list for his failure to past the conditioning test, to make room for the two players.
The Cowboys need help along the defensive line due to the injuries of four players.
Selvie, a 2010 seventh-round pick by St. Louis, has played for three NFL teams. He last played for Jacksonville in 2012, participating in nine games where he compiled 15 total tackles and one sack.
Cohen, a seventh-round pick from Detroit in 2008, played for defensive line coach Rod Marinelli when he coached with the Lions. Cohen has played in 25 NFL games with Detroit, Jacksonville and two stints in New England.