As suggested by @Sanjit_Pal you need to have different Login Profiles to achieve this. That said for Robot1 use Service_Account1 and for Robot2 use Service_Account2, this will allow you to configure multiple bots in same machine and also allows you to run concurrently based in your given schedule.
Edit : If you get a message as " already in use" which could mean that bot is already running in another machine, so once bot gets freed try assigning it to another machine.
Let me know if you get any error / challenges here.
Regards,
Rohith