You can and you should! No guinea pig should live alone. The main thing you'll have to think about is whether you'll go with a female (often easier to bond with a single boar) or another male (this avoids pheromones in the room potentially straining the bond between the remaining boar pair). You'll have to make a guess based on your setup and the personalities of your piggies, and then find a compatible pig in a rescue. Permanently having two groups may not be what you hoped for, but once a bond has failed, there's nothing you can do