It wasn't your fault entirely. You were ignorant, but your vet should have done research on his own to be able to support you. That's what he was paid for. You tried to do the right thing in the end.
If they were in the same room, there's a possibility of transmission. If they were in different rooms but you didn't scrub down/change clothes between handling them, the possibility is also still there.
If the other one gets sick, you won't end up killing her because now you have a better idea what to do. Go back to the vet, and get enough of the Bactrim to last for 2 weeks. It's cheap as dirt and very safe, so he shouldn't have a problem with that. Order some Critical Care by Oxbow to handfeed. It's a powder that you mix with water that has fiber and nutrients in it to help them keep going.
Check out the following pages for medical advice too:
You may also want to contact Texas Rustler's Guinea Pig Rescue, they're in Double Oak, which seems to be pretty close to you?
They should be able to give you advice on vets, since they've obviously got one for their piggies, and can provide more "on the ground" support. Nobody gets the knowledge out of the air, and I don't think you did a terrible thing. You did your best, and you did try to help your pig friend. Do you know how many pigs our rescue sees that are literally months into agonizing physical conditions, and yet the people refused to even take "just a guinea pig" to the vet?
It's not your fault that you didn't know, and you did what you thought was best. Now you do know better, and you can learn more and do better.