I just looked up that receiver to see what it is. It is two tuners built into one box. One tuner is controlled with an IR remote and the other is controlled with a UHF remote. You currently have the Slingbox IR blaster setup to control the IR remote of your receiver that is feeding the wrong tv. The obvious solution would be to switch which tuner feeds which tv. If you can't do that, you're going to have to find a way to drive the UHF remote with your Slingbox IR blaster. I don't know if there is a device of some sort to do this or not. Otherwise, maybe you need to get completely separate receiver boxes.
I just looked and there are some IR to UHF converters out there.