The best switches to use in your case would be Dish SW21 switch, they are proven to work with Viewsat receivers using Dual LNB's. It's possible that one of the LNB's isn't working properly. Best thing would be to test your LNB's by connecting both receivers to it without using a switch and see if they work.

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Please don't PM me with fta related questions, ask them in open forums so others can benefit from them too.