Is it possible to have multiple greater rift keystones at once
I have a Greater Rift Keystone that I use for running with my wife, but I also want to get one for when I solo.
Is it possible to have multiple Greater Rift Keystones on same character at once?
Yes it is perfectly possible. Every time you kill a Rift Guardian in a normal Nephalem Rift, you get a chance (up to 100% chance in Torment 6) to recieve a Greater Rift key stone. They stack in your inventory by level : up to stack of 100, after 100 you start a new stack. It also means you can have several Keystone of the same level at once.
Example :