OTA can't be diplexed onto MPEG4 sat cables.
To use the new MPEG4 sats you will need the AT9 5LNB dish which has four outputs. If all you need are two lines to the HR20, then only the two lines you have will suffice. However, for the OTA you'll need a separate line. If you need more than four lines, then the extra multiswitch (WB68) needs four lines connected.
There is a technology available called SWM (single wire multiswitch) which will help you. The 5LNB dish will connect to the SWM and then there is only one line into the house where it is distributed. You second line can then be dedicated to the antenna.
To use the new MPEG4 sats you will need the AT9 5LNB dish which has four outputs. If all you need are two lines to the HR20, then only the two lines you have will suffice. However, for the OTA you'll need a separate line. If you need more than four lines, then the extra multiswitch (WB68) needs four lines connected.
There is a technology available called SWM (single wire multiswitch) which will help you. The 5LNB dish will connect to the SWM and then there is only one line into the house where it is distributed. You second line can then be dedicated to the antenna.