Lord Visnu is is also jnown as Paramatma (supersoul) & we the minute spiritual living entities are known as jivatma. The jivatma originates from the Paramatma & thus has a spiritual relationship with Him. And also to add Lord Visnu is also known as Mukunda, meaning the "awarder of liberation".
Lord Siva is the greatest devotee of Lord Visnu (vaisnavanam yatha sambhu). He is never referred to as Paramatma or Mukunda. Lord Siva himself confirms the glories of worshipping Lord Visnu & His devotees in the Gita Mahatmya chapter of the Padma Purana.
Once Lord Siva got into trouble with Vrkasura who wanted to test the "head explosion" boon given to himself by Lord Siva himself. It was Lord Visnu, in a guise, who saved him from disaster.
Thus Lord Visnu is worshipped as the supreme controller & ultimate authority, and Lord Siva is worshipped as His greatest devotee.