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Yes, you can split the OTA signal. You just need to be careful how you do that.
There are three things carried by one cable in you diplexed configuration: OTA signal, satellite signal and DC voltage to power LNB and antenna pre-amp. The bottom line is that the signal passed to your HD Card should only carry the OTA component and nothing else. For example, the diplexer (at your equipment side) achieves exactly that: it separates the OTA signal from the satellite signal and it blocks the DC voltage to the OTA connection. So, if you you use a standalone diplexer (not attached to the back of the STB) you can easily split its OTA output.