In Sage it isn't easy to direct a specific program to a tuner but you can pretty much do this as you can enable/disable channels for certain tuners and you can set the relative prioirty of tuners.
In my system the OTA HD tuner is first priority, the SD tuner is second prioirity and the HD-PVRs are third and fourth prioirty. The OTA HD tuner only has access to OTA channels - on my system this is the ABC, CBS, NBC, FOX and PBS stations from Buffalo and CBC from Toronto. (If I wanted to futz around with the antenna in my attic I could likely also get the other Toronto OTA stations such as CTV, Global, CITY, but they don't broadcast anything I care about that isn't already available from the Buffalo stations). The SD tuner only has access to SD channels, both digital and analog. The HD-PVR has access to all HD channels, including cable only channels like TSNHD, TMNHD, etc., and access to all SD channels. Therefore Sage will only use the HD-PVR(s) to record SD shows if the SD tuner is already busy recording another show.
Sage is also smart enough to record a show in HD if it is available. You can also restrict Favourites/Series to certain channels (or you can record every occurrence on every channel). For example I tell Sage to only record 30 Rock on WGRZ-DT which is the Buffalo NBC affiliate which I have available OTA plus via my HD-PVRs. Since my OTA tuner is rarely busy 30 Rock will always record via my OTA tuner. That leaves my HD-PVRs free to record other content like Leafs and Raptors games that are only available on cable channels. And I tell Sage to only record Leafs games on TSNHD, LEAFSTVHD, CBCHD and SporstnetHD and Raptors games on TSNHD, SCOREHD, CBCHD and SporstnetHD.
I have had my system recording from all 4 tuners at once for the heck of it. The only problem is having the time to watch all of the stuff that I record - I usually end up deleting more than I watch. Who wants to watch a one week old hockey game? But it is great to be able to watch a hockey game starting at 10 pm, after the kids are in bed, and skip all commercials, intermissions, etc. and be able to watch the full game in one hour.