You should be able to go to a list of all your purchased titles. Then when you are on the page for the title, there should be a drop down menu with your TiVos listed. Then you select the TiVo you want to download it to. Of course you have to have set it up so your TiVos are on your Amazon account. I know there is a setup page where you enter your TiVo login and then it pulls the info from TiVo about the TiVos on your account.
Here is the Amazon help page for downloading to the TiVos
EDIT: I'm not sure if that page will actually help. I had to spend a couple of minutes searching to find the page for the TiVos. here it is.
On the TiVo page is where you can link your TiVo account and refresh your list of TiVo boxes.
Once the TiVos are linked and show up. Just go to a title you own and then you should be able to download up to two copies to different TiVos. And when you delete that copy it should free it up to be downloaded on another TiVo.