While I could not say that you would be committing sin by allowing the couple to stay overnight at your home – because there may be mitigating factors of which I am unaware – I cannot recommend facilitating an extramarital relationship by assigning the couple a single bedroom. If you have the space, assign them two bedrooms. Whatever they do outside of your knowledge during the night becomes solely their responsibility. Should you inadvertantly find out about nocturnal cohabiting during their visit, you can then resolve privately not to invite them to stay again in your home for overnight visits.
If you do not have two guest rooms, or if they refuse separate rooms and ask for a single, or if you have to refuse future overnight visits, you can simply say that you are unable to accommodate their stay in your home under such conditions – there is no need to be specific; they will understand the euphemism used above – but let them know that you will be happy to visit with them while they are in the area (which suggests that they will have to find a hotel if they cannot abide by the terms of staying overnight in your home).