It isn't TWO DIFFERENT PIPES. It is the SAME Pipe, but broken into sections. One section fell down beside the other and jammed into place. The only reason the 'ends' of the pipes (that were sticking out) are 'even' is because they were cut off by GIANT DIAGONAL CUTTERS.
Check out the Skandi One video feed. Great shot of the cap and riser uniting.
http://www.bp.com/liveassets/bp_internet/globalbp/globalbp_uk_english/incident_response/STAGING/local_assets/html/Skandi_ROV1.html