In root SSH, copy the /etc/domainips file and then remove the IP from it:
cp /etc/domainips /etc/domainips.bak110726
You'll see a line for the IP like the following that you'd remove from that file:
is the dedicated IP and domain.com
is the current domain using that IP.
After removing that line, then go to WHM > Resellers > Reseller Center > Add Reseller Privileges and make either that account or the other account a reseller (or create a new account first that will be the reseller owning both accounts).
After some account has reseller privileges, then back in Resellers area, "Change Ownership of Multiple Accounts" and select the reseller that should own the accounts that will be sharing the IP, and check the box next to each account that should be owned by that reseller. Click the "Change Owner of Select Accounts" button to switch ownership for those accounts.
Next, back in Resellers area, select "Manage reseller's main/shared IP" option, select the reseller you've created, click the "submit" button, then select the IP in the dropdown and click "Save" button.
At this point, you should now be able to go to WHM > IP Functions > Change Site's IP address area and select that IP as a new one for the other account. You should be able to share that IP for any accounts that the reseller will own.