See the problem is probably the HD receiver being left on a HD channel when you switch to view the SD receiver. Once the HD receiver sends out the +12v or +18v combined with the 22khz tone, the 3x8 switch can not be used for locals on that paticular polarity. I'm a bit surprised the 22khz went through the 3x8 for HD, but I guess since there is only one line affected it would.
Try this: With the 3x8 connected using the two satellite connections from the dish and to two outputs to the TiVo and one output to the HD receiver, tune the HD receiver to 101 or 201 (or any SD channel) to get to a channel that the receiver does not use the 22khz tone before switching to the TiVo. That should allow the TiVo to get the local stations. Now this will not be a solution that will allow the TiVo recording of locals unless the HD receiver is set to a SD channel. The ultimate solution is to run another coax.
For more info see: Connecting Multiple Directv Receivers