You should wait a month at least! Till you start introducing them, it takes a while for hormones to die down and if she still has any hormones she will get territorial with the new bun
So make sure you read all about bonding basically you can start pre bonding by swapping their living areas after your bun is spayed and healRd up then do the swapping for a month (put each bun in other cage daily so they are switched) and ideally they would be living next to each other so they csn look but not touch
Then aftet the month of pre bonding yoy introduce them in a neutral area a lot of people try the bath tub or bathroom floor and see how it goes. Only break up claw fighting little chases are fine
You should do neutral area meets for a while too. Ask any questions about bonding in the bonding thread too everyone is great
As for litter boxes two would be a good idea mine like to go in separate boxes across the room from each other