SOPS_AGE_KEY environment variable accept mutiples keys but thoses keys must be separated by new line. As an environment variable it would be more convenient to accept also any "whitespace" separator.
It's OK with :
$ export SOPS_AGE_KEY=$(cat age1.key age2.key)
But it's not OK with
$ export SOPS_AGE_KEY="AGE-SECRET-KEY-xxxxxxxxx AGE-SECRET-KEY-yyyyyyyyy"