Best Of

For example - create 2 HTTPS policies - 1 for WAN 1, the other for WAN 2
On the policy for WAN 1 - you specify in the From and/or To fields the traffic that you want to use WAN 1. The To: field can include IP addrs, subnets and/or FQDNs.
The policy for WAN 2 needs to be below WAN 1, and the To: field can be Any-external, the From: field could be Any-trusted for whatever is appropriate.
And you need to create a SD-WAN action for WAN 1 primary with WAN 2 as secondary and apply that to the WAN -1 policy. And do the similar for the WAN 2 policy.

Review this to understand how FDQN works in a From: or To: field
About Policies by Domain Name (FQDN)
https://www.watchguard.com/help/docs/help-center/en-US/Content/en-US/Fireware/policies/fqdn_about_c.html

Note that may sites use CDN (Content Delivery Network) URLs etc., so you would need to also ferret out those and add them to the appropriate policy To: field.
Turning on Logging on the WAN2 policy may help you locate those initially.
Also an Internet search for domain name used by selected sites (ie. YouTube) can help too.

Yes.
For A/P, just one firewall is active and holds the the external IP. This is done using VRRP.

Active/Passive Cluster ID and the Virtual MAC Address
https://www.watchguard.com/help/docs/help-center/en-US/Content/en-US/Fireware/ha/cluster_ap_cluster_id_wsm.html

For A/A, Multicast MAC Addresses is used to share the interface IP addr.

Find the Multicast MAC Addresses for an Active/Active Cluster
https://www.watchguard.com/help/docs/help-center/en-US/Content/en-US/Fireware/ha/cluster_view_multicast_mac.html

Also see:
Switch and Router Requirements for an Active/Active FireCluster
https://www.watchguard.com/help/docs/help-center/en-US/Content/en-US/Fireware/ha/cluster_aa_multicast_wsm.html

Will your switch allow 2 different subnets to be defined as on the same VLAN???

On the firewall, a VLAN has a specific subnet defined to it.
If traffic comes in tagged for that VLAN but from a different subnet, I would expect those packets to be denied as spoofed source.

It depends on the switch. Some may be perfectly ok with that, some may monitor traffic and refuse to pass it.

I would suggest for the sake of your sanity to put the networks onto separate VLANs if you can. The two networks won't be able to talk to each other (on the same VLAN) unless a static route is set up or machines on that specific network are multi-IPed.