I have had cats who were indoor and transferred to outdoor life quite happily and then back into being indoor cats. They are remarkably adaptable animals so that aspect I would not be too concerned about. As others have said, ensure they are neutered, vaccinated and microchipped before they take their first big steps into the wide world. Also I would be inclined to let each of them out separately so that you can keep a close eye on where they go. Then when they seem to be individually happy wandering around then both of them can go out together. It sounds like you are making a bit of a welter of it, doing it this way, but trust me it is the best approach. Once they have had a sniff around the first day call them back with a food reward, and gradually build up the time they spend outdoors.
I am assuming you do not have any major roads around. If you do, then perhaps you could consider retaining them as indoor cats ....