Presumably the driver is a buck-boost design which will take a 32 volt input and give you a 42 volt output. In principle, using a 42 volt supply with two strings of 21 volt each, in series, should be OK. The only uncertainty would be that the current for each string at 21 volt would need to be the same. The data sheet for the string would possibly show the range of current at 21 volt.
Without taking a lot of care with the string current at 21 volt, you may find, in series, that one string has say 24 volt and the other string say, 19 volt, and there may be a brightness difference between the strings. A brightness difference may exist anyway if you run two strings in parallel at 21 volt. It just depends on your application/specification.
You are certainly going to get a reasonable result; you might have to fine tune for brightness by using a shunt resistor across the string with the lower current(highest volt drop).
Hope this helps.