If both are captive born, have not been exposed to other tortoises or homes and are from the same breeder then there is little need to quarantine a new tortoise for a long period of time. However, it would be a good idea to keep the new tortoise seperate from the other one for at least a few weeks to allow it time to acclimate to it's new evironment and to ensure that it is eating well and drinking regularly.
I emphasis if it from the same breeder because this same advice does not apply if it is from a dealer or pet shop that often get their tortoises from various sources. Purchasing from either of those sources would definitely warrant a full quarantine period.