As long as finances allow, I'd say the sooner the better. If you can afford to provide for two pups (premium food, vet care, emergency fund, etc.) then my opinion is go for it
They can grow up together
Of course if you are going to get an opposite sex playmate, then making sure that at least one of them is fixed BEFORE a female might go into heat is extremely important, so that there are no oopsie pregnancies. Your vet can advise you on when the best time is to spay or neuter, but it's usually done around 6 months or so if they are above 3 lbs at that time. That's just a guideline though, there are many differing opinions on that subject
