Why there are 2 Spotlight instances ("Monitor" and "Monitor 2") connected the database?
Is it possible to switch one of them off?
No, it is not possible to turn one off. Spotlight uses two sessions so that long-running or frequent SQL scripts do not prevent a timely collection of data. It used to use only one session be we found that scripts like the one used for Fragmented Indexes often blocked the execution of other scripts for a long time which meant the data collected had gaps in it.