[libdefaults]
  default_realm = FOO.LOCAL

[realms]
    FOO.LOCAL = {
        kdc = foo-dc01.foo.local
    }

    TEST = {
        kdc = foo-dc02.test
    }

myLogin$ kinit -V  mylogin@test
mylogin@test's password:
kinit: krb5_get_init_creds: unable to reach any KDC in realm test, tried 0 KDCs

$ export KRB5CCNAME="FILE:/tmp/krb5cc_1000_prod" && kinit user@PROD && klist
$ export KRB5CCNAME="FILE:/tmp/krb5cc_1000_test" && kinit user@TEST && klist

$ kset() { export KRB5CCNAME="FILE:/tmp/krb5cc_$(id -u)_$1"; }
$ kset prod && kinit user@PROD && klist

runas /netonly /user:mylogin@test cmd