Short answer….If they are both genuinely saved…yes. However, Christians who shack up are violating clear-cut scripture on fornication and/or adultery and are giving a poor witness. Galatians 6:7 states, ” Do not be deceived, God is not mocked; for whatever a man (or woman) sows, this he (or she) will also reap.” Paul deals with the subject of sexual immorality in 1st Corinthians chapter 5 and specifically says in verse 5, “…hand this man over to Satan for the destruction of the flesh, so that his spirit may be saved on the day of the Lord.” God will try to get the attention of His rebellious children but when it gets to the point they will not listen, oftentimes His hand of protection is lifted to the point of allowing Satan to destroy them. I knew someone a long time ago that was an example of this. She was a married, God-fearing woman at one time, later had an affair, divorced, re-married, divorced again and lived in blatant sinfulness. She later joined the military and had a promising career but shacked up again and got pregnant. Shortly after having the child, she received orders for re-deployment but couldn’t take her baby to a potential combat zone ( this was right after the Cold War but tensions were still high ). She refused to go and was kicked out of the armed service. Shortly after that she developed cancer and died. God gave her over a decade to repent yet she refused and Satan was allowed to destroy her. Was she saved? Yes….but she lost much in this life and will lose much at the Judgement Seat of Jesus. A child of God who is living in blatant, open disobedience is playing a dangerous game. I hope this answered your question.