While there is an obvious need for IETF activity on underlying protocols, codecs and formats I'm unsure as to how a W3C RTC group is going to effectively manage any overlaps here. So am I. Input here and to the WHATWG and W3C on what people think would be appropriate methods to manage the division of labour would be helpful. I have my opinions, but I'm not the only one whose opinion matters.
The best suggestion I can come up with is to set up formal cross-review milestones. That is, the IETF RTCWEB might produce framework or API requirements documents that the W3C RTCWEB WG could review, and once there is agreement on those and things are further along, the IETF RTCWEB WG could review a W3C RTCWEB API document for compatibiity with those documents. Of course, this assumes that the IETF and W3C WGs work at a pace that permits this kind of cross-review to occur in a timely way. If the groups get way out of sync on timelines (e.g. the IETF RTCWEB WG complete all its milestones and closes, and the W3C RTCWEB WG is still in progress) then things could get tough. However, even in that (extreme) case, there would probably be interim drafts that the groups could look at. to make sure things were going in the right direction.