Do I need to replace both tracks or can I replace one at a time?
Very good question and there is no definitive answer, however you might like to consider the following few points.
Typically if one track breaks, the other track has done the same amount of work and is likely to have similar amounts of wear. So in other words, the 2nd track is likely not far away from breaking and is it worth the risk of more down-time?
Whilst operating mismatched rubber tracks is very unlikely to damage your machine in any way, if you are operating on steep slopes, the variation in traction from side to side could potentially cause stability issues. Just something to watch.
Is it worth changing both tracks and keeping the unbroken track in the shed for emergencies? You never know when you might need a track in a hurry to keep you working.
Lastly – changing tracks one at a time just looks wrong. If you are proud of your machine and look after it well would you really want to? I mean, would you walk down the street wearing a sneaker on your left foot and a Blundstone on the right!!