Trouble combining two variables in an iRule

Anyone tell me how to combine two variables in an iRule (BigIP HA pair, v11.2.1) ? Here's a partial of my iRule; it fails to work set tmpupn [findstr [ACCESS::session data get session.ssl.cert.subject] ",OU=XYZ" -10] set domupn "@domain" ACCESS::session data set session.custom.certupn $tmpupn && $domupn

 

It sets the tmpupn and domupn individually fine, and if I remove one, like the "&& $domupn" I get the tmpupn value.

 

I've also tried "ACCESS::session data set session.custom.certupn $tmpupn $domupn" and "ACCESS::session data set session.custom.certupn $tmpupn + $domupn"

 

Thanks for any help.