You cannot output two different channels from two different inputs on the TivoHD. The same channel is output through all connections.My question is, with the TivoHD, if I use that to replace both Series 2 DT and MythTV box, does it allow me to have multiple outputs? (ie, HD stuff obviously only goes out via components, but non HD channels can go out via something else)?
The component and HDMI connections use whatever resolution you have set in Settings -> Video. That includes, native, hybrid, fixed 720p, fixed 1080i, etc. S-video and composite connections always output at 480i. HD channels are downconverted to 480i through s-video and composite.
The TivoHD also has an aspect button on the remote to toggle between with three aspect mode. With your TV set to 16:9 under Settings -> Video, you get aspect modes full (stretch), panel (side bars), and zoom; when the TV is set to 16:9, you also get 16:9 anamorphic output on HD channels through s-video and composite. With your TV set to 4:3 under Settings -> Video, you get a different set of aspect modes, and SD is output as 4:3 through s-video and composite.