ANSWER: Those who believe in a Trinity carelessly assume the Bible teaches that the two Personages known as the Father and the Son are omnipresent - "everywhere present at once." It is the Holy Spirit that is Omni Present - not the individual Persons known as Father and Son.
King David asked God, "Where can I go from Your Spirit? Or where can I flee from Your presence? If I ascend into heaven, You are there; If I make my bed in hell [Sheol - "the grave"] behold, You are there. If I take the wings of the morning, and dwell in the uttermost parts of the sea, even there Your hand shall lead me, and Your right hand shall hold me. If I say, 'Surely the darkness shall fall on me,' even the night shall be light about me; indeed, the darkness shall not hide from You, but the night shines as the day; the darkness and the light are both alike to You" (Psalm 139:7-12). This Scripture plainly reveals that God is everywhere - but only in and through His Spirit.
God explained it this way: "Am I a God near at hand,' says the LORD, 'And not a God afar off? Can anyone hide himself in secret places, so I shall not see him?' says the LORD; 'Do I not fill heaven and earth?' says the LORD" (Jeremiah 23:23-24). God the Father does not, in Person, "fill heaven and earth." Jesus Christ, the Son of God, does not, in Person, "fill heaven and earth." Both Father and Son are omnipresent only in and through Their Holy Spirit. God - in Spirit - does fill heaven and earth!