At what point can we declare consensus?
Consensus is not always possible. If there is an equal, or almost equal, support for two contradictory positions, someone will have to make a decision. That someone will be the site owner (if applicable) and/or the moderators. So, instead of focusing on consensus, I'd recommend focusing on how to come to that decision and how to make sure (most of) the community accepts it.
You need to be aware and prepared for the risk that some users will not be staying with the site, no matter which decision you come to. But not making the decision will also drive some users from the site - namely those who don't enjoy the wrangling or the lack of clear guidelines.
So, here are some suggestions on how to make the decision and how to make it as non-disruptive as possible.
If you and the other moderators/owners are in agreement that you greatly prefer position A, you could announce it. A suggestion for wording here is something like:
"While both positions have obvious merits, it's not possible to have both A and B at the same time, and until we decide which we're going to have a problem with the community trying to do both things at once. The moderators have discussed this, and due to reasons X, Y and Z we find that position A will make it easier to keep the site working well. Therefore, we're updating the community rules according to position A. As we said, we know that there is also strong support for position B, and we wish it were possible to have both - but unfortunately it isn't."
If you and the other moderators don't really mind, or are also not in agreement, how about a vote? Give it a reasonable time frame and be very clear in advance on how you'll treat the result - will it be binding or a recommendation? What will you do if the result ends up 50/50 again?
Regardless of whether you make the choice yourself or allow a community vote, you also need to be clear about when and how the change will be implemented.
This is also a good time for you and the other moderators to consider how you're going to make these decisions in the future, and to communicate this to the users. It should be clear to everyone what the procedure will be if this happens again with another issue.