Hmmm, here in the US we do not vaccinate our buns.
They could have been having a reaction to a vaccination as well I imagine – it’s strange that they both did this though. I looked up Rear Guard, I don’t see any dangerous side effects with rabbits that are common.
Personally, I would call your vet and let them know what happened so they can prepare or at least be aware of this for next time. From what I’m reading, it’s a foam that covers the bum… I see you said soaked with… did they apply it everywhere? I’m not sure the consistency, you may be able to better advise on this, but maybe they both became cold from being wet from the foam? Rabbits temps can fluctuate very quickly AND easy. If they are ever wet, they should be wrapped in a towel or on a heating pad or dried off relatively quickly to ensure their body temps do not drop.
What about the possibility of them ingesting this? I would definitely follow up with your vet and let them know.