[RadioKid]

In traditional and sincere manner of writing letter, the receiver is supposed to be only one. Multiple receivers are not assumed.

If you stick to the traditional and sincere manner, you should prepare multiple cards for the receivers.

Or, you can send a card in casual manner of modern e-mail syle listing the receivers names at the heading.

In your previous post, you wrote "敬具" at the end of the letter. Please be informed that "敬具" needs corresponding "拝啓" before (and after the receiver's name) the letter.