One really here sees the different bridges and foremast - but one may call them twin sisters. despite being built at different yards and at different oceans (!!). Nice pic - I guess this is not an ordinary sight at Kingston-upon-Hull.
Although you can't see it in this picture, another difference is the car deck vents. Norsea (York) has the 'mushroom shaped' ones & the Norsun (Brugge) has a grid set in the bulkhead in the same area.
It is unusual to see both in together because they normally do the same run. What may have happened is one is covering the Europoort run whilst the Hull is in dry dock. (As the Brugges is on the Europoort berth, I'm assuming she is the Europoort ship).
this is when the pride of York (right ship norsea) went to to repairs it stayed in that berth for 1 week then went back into service on the Hull-Zeebrugge. The European trader covered the York while it was in repairs. the pride of bruge and pride of York can use all the ferry berths in Hull. the pride of bruge is staying on the Hull-Zeebrugge to next January then it will switch to the Hull-Rotterdam run for 1 week while the pride of Hull is in dry dock.