assign multiple shared IPs in WHM

You can add more shared IPs by following the steps shown below.

cd /var/cpanel/mainips

If there is no such directory, create it.

mkdir -p /var/cpanel/mainips

vi /var/cpanel/mainips/root

Add the current main shared IP and the secondary ip.

cat /var/cpanel/mainips/root
11.22.33.44
44.33.22.11

Go to WHM » IP Functions » Show/Edit Reserved IPs

You can see the status of new IPs as 'Main/shared IP for root'.

That's it :)