If your reservation is being made as part of a group reservation for a conference, of the type that usually comes with a discount and is made on behalf of the conference by a
travel agent or third party, you won't be able to get a dining plan at all. These types of reservations are not packages. They are room-only reservations.
To get a dining plan, except for a few very narrow exceptions (conferences are not one of them), you must be booking a regular package reservation including at least 1 day tickets through the Walt Disney Travel Company, or have a travel agent book it through WDTC on your behalf. Conference bookings aren't the right type of reservation. Unless the travel agency used by the conference is willing to book a regular package for you or change your reservation to a package, in which case you would probably forfeit any conference-related room discount. If the business is paying for the room, they may be more amenable to pay for the discounted room.
If you book through a travel agency, the travel agency must be the one to make changes to the reservation. You are not considered the owner of that reservation - the travel agency is. So while you could split the cost between cards if you are able to book a package reservation, you would need to do it through the travel agency (if your reservation is eligible for
DDP in the first place).
You could bypass the travel agency and book the package yourself but then I don't know whether your husband's business would pay for the room portion. Likely they aren't interested in paying for park tickets either.
So what you need to do is either you or your husband contact the travel agency and find out exactly what kind of booking this will be, and if it's not a package, if he has to stick to that booking to get the business to pay for the room.