If they were meeting for the first time at a one-day summer carnival:
400m: Shalane & Kara
1500: Shalane & Kara
5k: Shalane & Kara
Marathon: Shalane & Kara
Mountain Race: Shalane & Kara
If they had trained as much as they could prior to meeting:
400m: Usain & Oscar
1500: Shalane & Kara
5k: Shalane & Kara
Marathon: Shalane & Kara
Mountain Race: Shalane & Kara
~~~
Reasoning:
Whitlock too old.
Tough for leg/prosthetic adjustment between Oscar and Usain.
Difficult man/woman adjustment between Dean and Emma
Shalane & Kara -- fairly similar, both elite runners in their own right, that could easily compensate for their slightly-slower state as female runners with the fact that they are still incredibly fast ELITE runners, and bodies match more than anyone else.