Huh? Do you keep them outside?
In a wood hutch outside, in the day they go in a wired one on the ground so they can run about.
From what I understand, two males are usually fine together if they're both neutered or they've been together since they were young. Having enough space to share helps.
What did the squeaking sound like? Guinea pigs make a lot of different noises. If things are really bad though, you'll know. That's a scary scream to hear.
If you're checking when you hear it and aren't finding anything... it's probably not a huge problem. Better would be to check on both of them, and see if there's been any "barbering" or wounding going on.
As for there being poisonous stuff in the garden, that's definitely possible. There are a lot of common garden plants that we wouldn't eat or wouldn't affect us, but can definitely harm other animals. Do you know what your dog ate?
They have enough space, definitely. They have been together for a few months, but are young.
The squeaking is an occasional mild squeak then lots of chattering, then it stops.
The dog? No. It just stopped walking, went all shakey. One second she was fine, then she had to spend the night at the vets.