With FortiGate HA a-p mode, both unit has basically the same config (copied over from a to p). So if you set up a fail-over from wan1 to wan2 on the current active, the standby unit has the same config and duplicate the fail-over operation if a-p roles are flipped.
How let the standby unit take over the control from the active unit is a completely separate issue and you can control it by monitoring interfaces.
By assuming wan1-ISP1 and wan2-ISP2 are pairs, I would set up a fail-over mechanism wan1->wan2 with some optional split traffic first, then set up HA a-p between two 200Es. Much more complicated fail-overs are probably possible, but it wouldn't fit with HA a-p and more importantly wouldn't add much benefits in my opinion, other than more complicated troubleshooting/isolation process when a fail-over situation happens.