Hi czar,
Unfortunately you uploaded a jpeg. For short videos you can also use loom.com. 
But from your description I think you want to see/compare messages from two different topics at once.
This is possible with a little trickery. Since usually The READ() function is not bound to a certain topic, but to a payload structure, the values will change with every new message.
Comparing different payload structures:
It looks like you are using different payload structures for your incoming data. (1: actual, target, _timestamp; 2: cats)
If you just want to get rid of the “#NA” and keep older values alive, look into your READ() function and delete the “TRUE” in the “ErrorOnMissing” parameter. This parameter helps you identify “old” information.
Comparing equal payload structures:
If you need to work with the same payload structure from both topics, then there is another way to do so. Adding if statements. When you open the Metadata part of your payload you can use the “source” value to identify from which topic the message arrived. Either use the regular IF() function or even use the IF cells. If you now have a if statement + READ() function for every topic, these will only update for a message of the respective topic.
E.g.:
=IF("cell reference to "source" value"="topic name",READ(),FALSE)
or in If cell:
="cell reference to "source" value"="topic name"
I hope that is what you where reffering to
Cheers
Tizian