We were there in August of 06 and if I remember correctly, it counted as meals for two people (four credits) Overall, we weren't THAT impressed with Yachtsman, the food was OK, the service was terrible, definitely not worth the extra credits. Our steak at Le Cellier was much better!
Just to give another opinion (fair and balanced!), we did Yachtsman last Dec and had a fine meal and one of the nicest waitresses we have ever had. My husband is wanting to repeat it again this Oct, so they must have done something right. Enjoy! (oh, and yes, it is definately 4 TS credits for that meal).
Generally, any "two-for" item would either be automatically excluded from the Dining Plan (such as the Kitchen Sink at B&C), or would have to be ordered by two Dining Plan diners (as in this case).