Hi welcome to our forum. You can only use the rf output of the Foxsat-hdr to feed another satellite tuner under the following circumstances.
1 Foxsat-hdr is in low power sby. This will make all channels available to tuner 1 of a Sky box (which should be set to single tuner mode).
2 Foxsat-hdr is on, or recording, only channels in the same band and polarisation as the Foxsat-hdr tuner 1 is currently using.
For information on which band and polarisation Freesat channels use see
https://myhumax.org/forum/topic/what-can-i-record-and-watch-using-1-or-2-cables
Note the Foxsat can view other channels while recording two anyway – also see above thread.