
There is a 50/50 chance that the other cruciate will go.
The theory is that with proper and careful rehab, the second cruciate is less likely to rupture/get damaged. But most people let their dogs have too much exercise too quickly - and vets don't discourage them from doing so either.
It is important to rehab properly and not allow full off lead exercise until the operated leg is back to full strength - in other words until the circumference of the thigh muscle is the same again as the healthy one. That will take several months.
My dog had TPLO almost three years ago. He had both cruciates already damaged, it was decided to operate on the worst one first. This was done, I did physio with him for the first few weeks, planned walks and exercises to do with him very carefully and then introduced him to hydrotherapy and continued chiro treatment. At hydrotherapy they measured his muscles regularly and after six months, they were both the same size again and he was allowed to go back to normal off lead exercise. Now three years later, the other cruciate that was already damaged, still doesn't need repair and he is doing great. I put this down to his very careful rehab! All the other dogs I know that had the same surgery allowed their dogs to fully exercise much earlier again (usually at 3 - 4 months post op) - and they all ruptured their second one too!
I did a blog of my dog's recovery, let me know if you would like the link.