How can I share an external WAN interface between two VDOMs? For example, on a 100D, I wish to have two policies under different VDOMs send traffic to my WAN facing interface "wan1".
Short answer: No, I don't think it is possible to share a physical interface between two VDOMs.
For inter-VDOM communication, you can use inter-VDOM routing, ie. VDOM-link. You can find full documentation here.
It depends on what you want to achieve. If your goal is to have only one single external interface from the FGT to the internet, you can use the 'Management VDOM' configuration, which works even if you have more than 2 VDOMs. This older doc gives a really good explanation for the Management VDOM:
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