How to get 2 midsize dogs from San Juan PR to ST Croix ?
You may not be able to fly your 2 dogs in cargo due to heat restrictions in July if they don't meet in cabin size and weight.
If you do get to PR with them you can probably fly smaller airlines, possibly seaplane to STX.
See this link. Check airline rules for transporting pets.
Make sure none of your flights are on an Airbus plane.
They don't carry dogs in cargo at all, at any time of year.