I am no Thingspeak expert but thought to comment to test my own knowledge.
I have managed to get data from agent to 2 Thingspeak channels before and to do this you need to set up and use 2 write API keys (1 per channel).
I believe the alternative, which I could not get to work correctly, is to set up 2 fields within a single channel. According to the Thingspeak Channel Settings page you can set up to 8 fields.
I too attempted to create a string as you have shown for “field1” and “field2” but only field1 data was shown on my chart. So you not alone in trying to solve this…