Not currently possible. I'm also a necro and the multi-pet issue is a long standing problem SOE has yet to address.
Basically pet comands, pet spells and the pet window always apply to the oldest pet. I have yet to find any way to change from one pet to another. If for instance you charm something, then summon your main pet, you'll find it's impossible to heal your main pet since the heals always land on your charmed (oldest) pet, even if you manually target the pet you want to heal.
Note: This bug does have advantages that are deffinetly not intended, so I won't mention them
