It is impossible to guarantee the gender of any baby tortoise. It is pot luck until you get a definite ID aged 5-10 years.
Any companion for your existing star must be kept entirely separate from yours in quarantine for a good 6 months.
After that, there is no guarantee your star will want to share accommodation or food. As they grow older, particularly if they both turn out to be male, you may well have a fight on your hands. Territorial disputes can occur regardless of gender... And the loser can become withdrawn, ill and possibly die.
Please only get a second tort if you can accommodate them entirely separately and are prepared to accept that this may have to be permanent.