Hi Frank,
When I've added '1:1' relationships in the past, I remove the links in the left hand navigation and create an inverse relationship that I update with workflow onchange of the other relationship (lookup). Effectively, you end up with a lookup on entity A
that points to Entity B and a lookup on entity B that points to entity A, allowing the user to navigate between the two records.
You don't mention the process by which your users will set this relationship so assume it's set from just one of the 2 entities. If it can be set from 'either side' of the 1:1 then you'll need 2 workflows (trigger on change of each lookup) that has some
clever logic to avoid an infinite loop.
I hope that helps/makes sense!
Rob
MCTS. GAP Consulting Ltd. Microsoft Community Contributor Award 2011 & 2013