We're in the midst of our peak vacation boarding season at the rescue, which means pigs coming in and out of Cindy's house every weekend. We usually have a few people each summer that we need to educate on the subject of "safe guinea pig travel" -- though the number gets smaller each year (happily).
You need at least one travel carrier for transporting your guinea pigs anywhere outside of the home, including to the vet and to wherever you board your pets while you're on vacation. Transporting guinea pigs by any other means, such as cardboard boxes, plastic storage boxes, cages, or in a towel in someone's arms or lap is not safe. Sudden stops or other sudden movements in a vehicle could cause injuries to a guinea pig if s/he slides around in an unsuitable carrier.
Any hard-sided carrier that is marketed for cats is large enough to accommodate a pair of full-grown guinea pigs, without being so large that they could slide around as much as they would in a box or cage. (We do not favor soft-sided carriers for small critters.) Carrier floors can be lined with towels (or puppy pee pads) to give your guinea pigs' feet some traction, plus a towel/fleece pouch/cozy cup for them to curl up in and stay warm.
The Petmate Portable Kennel has secure locking doors on the top and front of the carrier. The top opening makes it easy for you to slip food into your guinea pigs while they're in the carrier, and to check on them in the car without risk of them escaping.
The Small Kennel Cab has a secure locking door on the front of the carrier. The carrier provides only one way in and out.
Geared more toward small critters, the Cabin Kennel has a slide lock on the side and opens from the top. The wire vent on the top of the carrier provides a small door that you can slide back so that you can slip food into the carrier, or even your hand so you can check on them during travel. This carrier isn't ideal for longer trips, but it will be sufficient if the only road trips your pigs ever go on is to the vet.
The cat carriers work for ferrets, bunnies, and chinchillas, too.