2 node Exchange 2013 Mailbox/CAS servers. I am renewing my SSL cert, do I need to install same cert on both servers?

Yes, just like you should already have the current certificate on both servers ;-)

Step 1: Obtain an SSL certificate. Purchase an SSL certificate from a well-known certification authority (CA).

Step 2: Generate and submit the certificate request: create a new certificate request for Secure Sockets Layer (SSL) 

Step 3 : Import the new certificate

Step 4 : Enable the new certificate  

You can also follow my write up on the same

http://exchangequery.com/2014/12/12/steps-to-perform-ssl-certificate-renewal-in-exchange-20102013/

did not perform original install