There are two circumstances where you can use 903 while recording two, the second being very unlikely
1 The two recordings are being made from the same transponder, eg BBC-HD and BBC1-HD. This combination allows access to any other channel
2 One of the recording pair is channel 407 Fashion One (same transponder as 903).
See
http://myhumax.org/forum/topic/what-can-i-record-and-watch-using-1-or-2-cables
Although the link only mentions the Foxsat-hdr it applies equally to the HDR-1000s