I apologize if this has been answered before but it is hard to search 2 tivos without coming up with series 2 tivo, etc.

We just "acquired" (stole) a tivo from our in-laws that was "broken" (user error). Is there a way to program my Pronto 3500 so they don't both respond to the same remote?

The standard way to achieve this is with a RF extender stuck to the front. Make sure the one using RF has the IR window blocked so it wont accept IR as well and then have your 2 devices in your pronto set to IR and RF for the different Tivos.

Even easier would be to have the one controlled via RF using the extender in a cabinet so it wont receive the IR direct from the pronto.

Most decent remote layouts have the IR codes in seperate page/devices and links setup to the visible pages. This would make this really easy to achieve simply by copying your IR device for the Tivo and then setting one to use RF and leave the other on IR

I don't specifically know about Tivos. There may be a way to switch remote codes like my Sony amp which would be even easier and require no extra hardware

There is a switch on the Tivo remote and I was able to set up the Tivo remote to control the units separately by changing the switch. So unit one is set to remote address 1 and unit two is set to remote address 2. But I have no idea how to switch the remote address on the pronto. Is there a discrete code for this? The way the Pronto is set up now is as if it is remote address 0 because it will control both Tivo units.

I guess you will have to learn the codes to 2 separate pages with the remote set to each different setting

You can use MakeHex to generate clean code sets for any of the Tivo addresses. MakeHex might be easier than learning; and clean codes might function more reliably than learned codes.

As the post above implies, there is nothing in the Pronto that acts like the switch you might want to allow one panel to be reused for more than one code set. However many panels you needed for Tivo control, you would need twice as many for two different addresses of Tivo control.