Shouldn't be too close, especially if you are looking towards the end of June. It used to be a set-piece before a BGR just to confirm winter training had gone well.
In 2006 I did a 24:08 BGR and followed it with a 23:39 round 7 weeks later so you should be able to recover quickly enough. It may be that time spent on the Fellsman would be better spent on the BGR route and that is more difficult to dispute.
Whatever you decide, enjoy your BGR.