Samsung has one of the lowest SAR results, compared to other smartphone manufacturers.
Slot 1 is default as you probably know.
With 2 sim cards installed and slot 2 disabled yes it will stop slot 2 from functioning in theory however it may still carry out backround processes, during OS updates, booting up or down etc. Although visually disabled the phone will know the sim cards installed, The best method would be to just remove the sim. You could go in to settings and check your active backround processes and try disabling it that way. Slot 2 disabled with sim card installed will only give of limited radiation.
For protection or reduction from radiation to yourself, try using your speakerphone or an handsfree kit, effectively keeping the phone away from yourself. Low signal can increase radiation output. Finally you could buy a case to reduce the radiation.
Interferance wise switch off inactive functions, turn on data saving.