Exactly. If I can interact with the other then I could just tell them what they're supposed to experience and we could both lie. Or we would color each others subconscious minds enough that we would dream approximations of what was discussed. This would be akin to having a non-lucid dream about being lucid.
Another problem lay in the fact that it may take some sort of connection with the other. To address this issue I thought that maybe there could be a different groups:

Group one would have intimate knowledge of each other through supervised sessions. These would be able to interact as much as they please, but not discuss dreaming, thus eliminating overt collusion. This group would be able, in theory, to come up with some sort of sneaky cheat, but with strict supervision it could be greatly discouraged.

Group two would be able to interact only with a medium/mediums of the person they are to attempt sharing with. Maybe photos, video, and recordings of speech. I think the video recordings might have to be sans audio and the audio recordings of some predetermined questionnaire that all of the participants are made to fill out.

Group three would be given no contact with any of the participants, only take part by filling out a dream journal to be checked against all participant's DJ's for similarities.

The only time I'm sure of having shared a dream was with my sister, who was living quite a distance from me at the time. It had been awhile since we had interacted more than over the phone, and neither of us was actively practicing, although we have both recorded LD's since we were young. For that reason I believe that, although contact may make for more consistent dream sharing, it may not be necessary for DS to occur.