In the case where you're "in business for yourself" and don't actually have employees who can act as a second photographer, then I think one of the fairest ways to the couple would be the following:
- Ask them up-front if they expect/want a second photographer.
- If they say "yes," then tell them that you have a few different people you use (assuming this is the case; if you only have one person, then say so ...) and that you will contact those people / that person immediately to verify their availability.
- Do that - call your backup several of your backups to make sure they're available for that date. If they're not another wedding photographer, then it's much more likely they'll be free and remain relatively free within the time it may take you to get the couple to book you.
- Inform the couple that either (a) yes, you have one or several different people who can help you for their event, or (b) be honest and say that everyone you know is already booked. If (b), then you can try to push the sale by explaining your experience with large weddings, etc., but be honest and fair to them and say that if they really want two people, you won't be able to help them.
Those steps may seem like a little extra leg work, and you could emphasize to the couple at the first meeting that you've always been able to get a backup (assuming this is true), but I think it's more ethical to do it that way than to tell the couple that you absolutely can get someone when you haven't spoken to backups yet.
Another possibility, assuming that before you meet the couple you have the basic information like the date of the event, then you could call your backup(s) before the meeting so that you can go into the initial meeting and say something to the effect of, "If you want there to be two of us, I already have someone who can act as a backup and I've ensured they're free on [date]."
Assuming then that you do have a backup, let them know as soon as possible that the couple has booked. If the backup is "on your list" as someone you've spoken with before (which they should be if they're on your list), then you probably already have some sub-contract worked out with them.
I would think that the terms of the sub-contract would be fairly universal with only the monetary split being most highly variable. That should probably be based upon your backup's level of experience.
Good luck figuring out what you want to do here, and I hope I didn't shoot myself in the foot (being someone who's contacted you about being a backup

).