The general rule is that all occupants of the room are treated as if they're there for the full length of stay. So you'd pay for (and get the credits for) all three members of the party for the full time. However, the credits can be used by anyone in the room (normal adult/child restrictions apply, of course).
The other, and likely preferable, option is to do a "split stay", where you make multiple consecutive reservations, each potentially with its own dining plan. The allocated plan credits are tied to each reservation -- you can't carry them over from one to the other (except for some overlap on check-out/check-in days).
Whether either approach makes sense comes down to a few factors:
1) If you're staying at DVC on points (your own or renting someone else's), you can book multiple "segments" for the same reservation, each with its own dining plan options, and even for different numbers of guests.
2) If you have (or will be getting) an annual pass, then making multiple reservations with separate dining plans should be fine because you won't need to buy multiple tickets for each guest in order to get the normal "package" necessary for a dining plan.
3) If you're going during free dining, stick with the other option -- the cost of adding your husband to the reservation (minimum 2-day ticket) is there whether he stays 1 week or 2, and you can use his credits even in his absence.
