A Reserved Instance a method of pre-paying for Amazon EC2 capacity.
If you were to buy two Reserved Instances (in your case, 2 x t2.large Linux), then for every hour of the year while the Reserved Instance is valid you will be entitled to run the matching instance types (2xt2.large Linux) at no hourly charged.
There is no need to find which instance is the Reserved Instance. Rather, the billing system will pick a matching instance that is running each hour and will not bill any hourly charges.
Therefore, if these are the only matching instances you are running, then they will (by default) be identified as Reserved Instances and will not receive hourly charges. If you run different instances, however, there is no way to control which instance(s) receive the pricing benefit.
It is possible to purchase a Reserved Instance with, or without, identifying the Availability Zone. If an AZ is chosen, then the pricing benefit of the Reserved Instance only matches an instance running in that AZ, and there is also a capacity reservation to give you priority while running instances that match the Reserved Instance. If no AZ is selected, then the pricing benefit applies across any instances running in that region, but there is no capacity reservation.
Yes, it will apply immediately and there is no need to start/stop/rebuild anything.