Which way should I add a Second Secondary?
Should I add a second secondary to my current RDS or do I need to have 2 RDS's for my Primary?
If want to replicate from one
If want to replicate from one primary to 2 secondaries - then this is one RDS so you should add secondary to your current RDS. If you have 2 pairs of Primary and secondary (i.e 2 different primarys) then this is 2 RDS.