Hello I am currently installing a closed loop CM Tranquiliy 27 in New York about 60 miles north of NYC.  Is it recommended to have a electic strip back up in the plenum for extremely cold days?  I am also planning on keeping our existing oil furnace as a back up if the CM 27 ever fails.  Thank you for the information.

It all depends upon your heat load. Have you had a manual J load calc done? How are you going to use the furnace for second stage

Sorry not second stage, supp/emerg heat

The installer had a basic heat annalysis done. We are working on how we want to use the existing furnace.

If you are keeping the existing furnace (Along with its oil tank, standby / stack losses and annual maintenance for safety and efficiency) then no, you probably don't need backup strip heat.

Unless you suffer frequent/extended power outages, there is little need to leave the existing furnace. Many people approach geo with a wary eye, yet failures and maitenance will be less than your oil furnace offered. What system did you use to back up the oil furnace?
If the oil furnace does stay, you should look at a split system geo and employ electric aux. strips as well (if they fit) as operation with supplemental electric is cheaper than shutting off the geo and going to oil.
Did your installer recommend leaving the old unit? What was the heat load and what size heat pump are you installing?
I would not encourage paralell systems use a TS (CM split system) or do away with the old unit.
Good luck,
Joe