Create a traffic selector '[two sites] -> Any -> Any' and use it with a Bandwidth pool on the Internal interface guaranteeing 90% (for example) of your WAN download bandwidth.
Since big uploads also can block HTTP requests, you might want to put another bandwidth pool on the External interface guaranteeing, for example, 100kb to 'Any -> Any -> [two sites]'.
If each site needs to be defined as a DNS group, then be aware that you cannot do a group of groups, and you'd need separate traffic definitions for each.
Create a traffic selector '[two sites] -> Any -> Any' and use it with a Bandwidth pool on the Internal interface guaranteeing 90% (for example) of your WAN download bandwidth.
Since big uploads also can block HTTP requests, you might want to put another bandwidth pool on the External interface guaranteeing, for example, 100kb to 'Any -> Any -> [two sites]'.
If each site needs to be defined as a DNS group, then be aware that you cannot do a group of groups, and you'd need separate traffic definitions for each.