While the books I purchase from Amazon stay in sync, I sometimes add books to the two devices that I got somewhere other. from the Home screen, at the top, press that little gear icon to enter Settings select More (the plus icon) from the long list of stuff, select Applications in another long list of stuff, find and select the Amazon Kindle app Now we have several options: Force Stop the app, clear its data, or clear its cache. The fact that Amazon don't allow you to set up an email address for any non-Kindle device (and I have the Kindle app installed on my laptop as well as my phone) suggests to me that they're keeping the conversion service as a "value-add" incentive for people who've bought a Kindle device. I have the Kindle app on my iPad as well as a real Kindle e-reader. I do this from Andoroid Settings - Applications - Manage Apps.
The upside of this for Kindle owners is that you and other family members can both have Kindle's on the same account (and thus share books) but you can avoid bombarding anyone else with your documents (if you did want to send to both at the same time, you just list both addresses in the "To" box when you send the email). My Android came with the Kindle App (version 1), so I dont uninstall the app, only the updates. If you had more than one Kindle device, then you can assign different email addresses to each. If you look at your Manage Your Kindle page on Amazon (the same page where you set the email address up) you can see that the Kindle email address is specifically assigned to one Kindle device.