I think a fairly simple and realistic buildup would be when he comes to get his van, D/F and him go to a restaurant or something and Martin lets D drive the van (didn't she already do that?).
- Go to a restaurant (to chat, catch up with each other etc.)
- *Choice* D drives the van
- *Choice* Perhaps Martin sits in the backseat behind D and puts his hand on her shoulder to encourage her.
- *Choice* D and or Martin flirt (about the driving skills of D for example)
- Martin spends the night, which would lead to the sharing if the player didn't reject the previous choices.
Maybe they even run into Olivia/Graham there and have a quick chat, if you want the sharing paths to overlap some.