Good work you are doing here. Since it is a free sevice, there is little I can complain about. I think is not wise to develop the same thing with the same functions using different technologies. I am sure your team must have has a lot of discussions about it. Take it from me, it is a VERY VERY BAD idea.
It is bad enough that there are too many forums not to talk of one company with two. I am sure you guys have seen a reduction in the rate of downloads of PUN BB.
Just take a second to imagine a company having another complete factory to make the exact same car with a different material. Or Imagine Wordpress deciding to use have a paralell blog system in a different language doing the same thing. You cannot tell me you believe it is a good decision. Listen to your users.
There might be commercial reasons for the .NET but you can decide to abandon the PHP and move full speed ahead and those who want PHP can move to fluxBB, and those who want .NET can stay.
You have Facebook using your forum, you can Capitalize on that and make PunBB the greatest forum ever made. Do not destroy your own success.
If you want proof that PunBB.NET is a distration, this forum is it. The time that would have been spent thinking of how to develop PunBB is used to talk here. Tell your sponsors it is a VERY BAD idea. I leave you with one advise and it is the Acronym of FOCUS. Follow One Course Until Successful!