I try to use map directive. I have 2 vars and if first var exist set it to new var, or if only second var exist, set it to new var. My config:

map "$arg_arg1:$cookie_1" $new_var {
"~^.*:" $arg_arg1;
"~:.*$" $cookie_1;
default "new";
}
Work only in first situation.

nginx

Working config:
map "$cookie_1:$arg_arg1" $new_var {
default "new";
"~:.*$" $arg_arg1;
"~*^.*:$" $cookie_1;
}

Did you resolve this by yourself ? This is advanced nginx configuration.