Assuming that you have both NGINX and Apache installed...

1. Select different IP addresses for each one.

Let's setup the hosts file for quick access to start pages.

sudo nano /etc/hosts

append lines (use any local IP you like)

127.0.0.1   nginx
127.0.0.2   apache

2. Setup listen IP and port for NGINX

NGINX must listen on one IP address only.

sudo nano /etc/nginx/sites-enabled/default

And replace the lines

--- (remove lines) +++ (add lines)

--- listen 80 default_server;
--- listen [::]:80 default_server;
+++ listen nginx:80;

If you want to use SSL, make the same things for 443 port.

IMPORTANT!

Make sure all enabled NGINX websites listen on nginx:80

Restart NGINX

sudo service nginx restart

Make a check using command sudo netstat -tulpn | grep :80

tcp        0      0 127.0.0.1:80            0.0.0.0:*               LISTEN      26540/nginx: master

Done! Now you can access default NGINX host by url http://nginx

3. Setup listen IP and port for Apache

Apache must listen on one IP address only as well.

Ports:

sudo nano /etc/apache2/ports.conf

And replace the lines

--- (remove lines) +++ (add lines)

--- Listen 80
--- Listen 443
+++ Listen apache:80
+++ Listen apache:443

Default virtual host:

sudo nano /etc/apache/sites-enabled/000-default

And replace the lines

--- (remove lines) +++ (add lines)

--- <VirtualHost *:80>
+++ <VirtualHost apache:80>

If you want to use SSL, make the same things for 443 port.

IMPORTANT!

Make sure all enabled Apache websites listen on apache:80

Restart Apache

sudo service apache2 restart

Make a check using command sudo netstat -tulpn | grep :80

tcp        0      0 127.0.0.2:80            0.0.0.0:*               LISTEN      26829/apache2

Done! Now you can access default Apache host by url http://apache