Try this way
"[" & field1 & ", " & field2 & "]"
When I build code for Geoprocessing tools, I would also check the documentation to see how it's constructed in python, then mimic that behavior.
For the Frequency tool, it's looking for this syntax
arcpy.Frequency_analysis("taxlots",
"C:/output/output.gdb/tax_frequency",
["YEARBUILT", "COUNTY"],
["LANDVAL", "BLDGVAL", "TOTALVAL"])
