Gurudev, even though there is no difference between Lord Vishu and Lord Shiva, who originated from whom? The Vishnu Purana glorifies Lord Vishnu and the Shiva Purana glorifies Lord Shiva.
When you think, who originated from whom, you are thinking on linear terms. But truth is not linear, it is spherical.
That is why this is also true and that is also true. From which side you see, that is where it comes from.
If you see from that side then that is true. If you see it from this side then this is true. But both are same actually. This is what spherical thinking is.
It depends from where you start and where you go.
Shiva and Vishnu are different, yet both are the same.