Two dogs from separate families?

I'm sorry your first experience with hosting dogs from multiple families has turned out so stressful. In the future, I would recommend introducing the guest dogs without your own dogs present and slowly expanding the group as everyone is comfortable with the new dynamic, and be prepared to enforce time outs when one of the dogs gets uncomfortable or starts behaving inappropriately, especially if one of your own dogs has a tendency to be pushy or rude with other dogs. "Showing them who's boss" is bullying, and like with human interactions, it is likely to cause problems. Many dogs may put up with it, but that doesn't mean it's acceptable behavior. A less confident, more appeasing dog, like the heeler, may be very intimidated by the bully and fearful of standing up to them; he may have redirected onto the lab because the lab is more docile and less frightening. You see the same thing with bullied kids: the bully picks on someone who is afraid to stand up to them, and their victim takes out their anxiety and frustration on another, safer target.

As for your current situation, just do your best to split time and attention between the dogs. Take them for walks separately, play with them separately, spend your down time alternating between them. Rotate them through the areas of your house, and when possible, leave them with toys/treats (like a long lasting chew, puzzle toy, or stuffed kong) to keep them occupied when you can't be with them. It's not ideal, but if you're able to keep them safely separated until Wednesday and you can provide for their basic needs, that may be your best course of action. You're certainly encouraged to inform the owners of both dogs of the situation in case they would like to make alternate arrangements. Let them know that there was an interaction between the dogs that scared the heeler, who is now defensive around the lab, and for their comfort and safety you're keeping them separated and attending to their needs individually for the remainder of the stay. Tell them what you told us: both are sweet dogs and you really enjoy them, and you just want to make sure they're both cared for and comfortable. For the heeler's family, you might let them know that in the future he may be more comfortable with a sitter who doesn't house multiple dogs, since he may be a bit overwhelmed with a larger group when his family isn't around.

Honestly, I would keep them separated. If you have experience with correcting bad behavior, you might try reintroducing them. If decide to give it a go, I would put your dogs up while doing so. It sounds like your poodle might've set off the heeler since neither dog had an issue with one another prior to the poodle's reaction.

I do not recommend you reintroducing them if you do not have experience with correcting bad behavior.

Well done. Good solution. Forgive the humor, but the wording of your original question brought to mind the prologue from Romeo and Juliet: Two households, both alike in dignity, In fair Verona, where we lay our scene, From ancient grudge break to new mutiny.... I know. I'm a nerd.