Vishnu is all-pervading. What this means is that he is ever-present, throughout all time and space, sustaining all of nature. In this, Vishnu's immanence consists in his occupying a much more intimate relationship with time and creation than is Shiva's - the grand annihilator. This is not to say Vishnu is not transcendent. He is transcendent in that within him the principle of space and time has its condition, whereas Shiva is fully transcendent in his aspect.
One may in this regard paint the situation as a triangle. The top corner sees Shiva, calm and absolute, as presiding over all of nature and passing beyond it, into the fullness of nameless eternity. On the bottom lies Vishnu, the all-pervading purusha, stretching infinitely over all creation and containing it: all times and all localities lying within his compass. This is hardly to say Vishnu is not also nameless and transcendent, however. For what does it mean for the one substance, Hari, to be extended over all creation, to be simultaneously omniscient and purely cognizant of all things, if not to delve into the utmost transcendence?
In point of fact, both Vishnu and Shiva are one. How can they be separate? It is not merely in terms of what is apparent. Vishnu and Shiva are indeed distinct personae of Ishvara. Their oneness consists in their absolute character. How is this jarring contradiction to be resolved?
One requires a higher logic. One that takes into account the paradoxical nature of time and amends it. It is then seen that God can be more than one, in degree though not in kind, by virtue of his bearing a relationship to time differently. Shiva lies above time, whereas Vishnu possesses it. The one is aware of time to such an extent that he surpasses it completely. The second is as acutely aware as the former, but he prefers to strive within it and accomplish something out of time. The latter attempts to work and suffer in the context of the created order, to grow in loving ambition and orgiastic evolutes, to which Mahesh can in no wise sanction for his own discrete person. This endeavor is reserved for Narayana, the true indwelling "Man" in each of us, as he learns and suffers with us the pangs and groans of the whole created order yet remaining, inconceivably, ever the Master and eternally wise Counsellor, in his own infinite and ever-Supreme Moment.