Two iPhones receiving same SMS when sent or received
To remove your work iPhone 6 from your personal iCloud account, go to Settings, select iCloud, scroll down to the very bottom, then tap Sign Out.  

To disable iMessage on the work iPhone without removing it from iCloud, go to Settings, select Messages, and uncheck the iMessage option.  

If you're receiving weird, spammy iMessages, scroll down to the Send &amp; Receive option of the Messages section, select that option, and check/uncheck the addresses you'd like to receive iMessages from.  iMessages can be sent to email addresses, so if you have a public-facing email or one that's shared often, you might want to uncheck that address.

If you're not receiving text messages even after disabling iMessages, you may wish to use Apple's Deregister iMessage page; it's intended for people who are switching from an iOS device to a non-iOS device, and thus wish to disable iMessages.

Another tip: Make sure that the people whose messages you're not receiving aren't attempting to send iMessages to your personal iMessage account!  Make sure to give them your work phone number, and text them from your work phone; otherwise, they may have created a contact with your personal iCloud account as the iMessage address.