Putting both shoulder straps into one mounting point is just daft.
The lap belts are easy, one in the stadard point and drill and plate the other in the exhaust tunnel.
As per the MSA Blue book, page 166 of the book, part 2.1 onwards, also ref page 189 of the book for a diagram.
If the gouverning body of motorsport in the Uk deems that safe, that will do everybody in the UK regardless of what tom, dick or harry tells them.
http://www.msauk.org/uploadedfiles/msa_forms/bluebooks/10/160-195 Competitor Safety.pdf