Nginx проксируют алфавитом

Yeap, everything's possible:

I presume you don't actually mean redirect when you say "redirect":

server {
    listen 80;
    server_name     example.com *.example.com;
    proxy_set_header        X-Forwarded-For $proxy_add_x_forwarded_for;
    proxy_set_header        Host    $host;
    if ($host ~ "^[a-o]") {
        proxy_pass http://2.example.net:80;
    }
    if ($host ~ "^[p-z]") {
        proxy_pass http://3.example.net:80;
    }
    return 404;
}