configuring HA with 2 fortigate 100D and 2 ISP links.

Basically, look at the HA cluster as if it were ONE Fortigate. The feature you're looking for to address 2 ISPs is called "ECMP". There is an article for sure in the Cookbook to show you how to configure this. See docs.fortinet.com.

There might even be videos on this topic (video.fortinet.com).

In newer versions of FortiOS the feature is called "WAN load balancing".

Just be aware that for each port you use you need a switch with at least 3 ports (FGT1, FGT2, destination). If you use more than 2-3 physical ports, you can use a 24port switch and segment it into smaller switches by using internal (port-based) VLANs.