Addition of 2 parameters in Datalink

Hi all,
I have 2 tags coming from 2 different devices of MBE driver. I created 2 tags in iFIX database. I'm trying to add both the values using formula in datalink stamper. If any of the device communication fails, I'm getting '?????'.

I would like to display the value of sum of two tags if both the tag communiction is helathy and if any of the tag communication fails, it shall display the value of the healthy tag.

Thanks in adv,