Planning to breed Spur thighs one day:?:

[NickyF]

My daughter and I have just bought two beautiful 3yr old spur thighed tortoises (Cleo and Roy). Our plan is one day in the future, I guess about 4/5 years to try to breed them but need to plan ahead. My daughter will be taking 'Roy' with her when she moves away at the end of the summer so our plan was to both buy a second tortoise to keep them company and to bring on for breeding when old enough.
Our problem is twofold!
1. uncertainty with sexing 3 yr olds and younger
2. can tortoises live happily together or will the males fight if we end up with more than one male

What age can we be sure of correct sexing?
If we were able to have a male and female each, would this be the best option?

We would be so grateful for advice as we are very excited but seem to be going round in circles trying to make the most sensible plan.