I have a similar setup but with 3 decoder boxes plus a DVD/HDD recorder, and one of the decoders has HDMI, so one more than you have, anyway this is how I would do it:
Decoder 1 scart to TV Scart.
Decoder 2 scart to TV Scart
Connect recorder to TV using HDMI
Decoder 1, 2nd scart output to recorder.
Decoder 2 2nd scart output to recorder.
With this you will be able to watch all sources, and record from both sat receivers.
Robbo