Goal:

With Connected Coasters, each friend has a linked coaster, when one friend is having a drink and they set a glass on their coaster, both coasters will light up with one color, when the second friend joins in and puts their drink on a coaster, both coasters will light up with two colors, signaling that the two friends are drinking simultaneously. 

This solution includes identical coasters which will allow both friends to be notified when their friend is having a drink and when they are drinking at the same time, giving a sense of connection and bonding to both parties.