Is there any truth in Private Eye's observations on p29 of issue 1204? Is it normal to have redundant software written by different companies in the commercial aerospace industry?
It is not the norm at all. In fact it is very much the exception to the norm. Typically, most systems will have more than one channel (think of it as a primary channel and a back up channel) on separate cards. Typically there would be some health checking between the channels with the primary staying in control of the system unless it is "less healthy" than the second channel. In most cases, the software contained in each channel is identical. In rare cases, the software will be different (referred to as Multi Version Dissimilar Software in RTCA/DO-178B - the guidance to which the vast majority of modern aeronautical software is compliant).