You give them a bath together. It's scary for most guinea pigs so they turn to each other for safety and build a friendship. You can use the sink but the method that I found works is the bathtub. Get the water to the right temp(warm but not hot) and set the 2 pigs in the back of the tub. Run as much water out of the faucet as possible so it's noisy then plug the drain. When you have a few inches of water turn off the water and pull the guinea pigs into the deeper part. Pour some water over them and if you want you can use a little pet shampoo to clean their rear ends and tummies. Rinse, dry, and wrap them in a towel together. Set them in a pen and they should bond. The more they are next to each other the more they more likely they are to become friends. So far it's worked with 2 very agressive males that drew blood on both their cagemates.