God can be everywhere at once because He is God and can separate Himself into Father, Son and Holy Spirit.
The Psalmist David told us, "If I ascend to heaven, you are there, and if I make my bed in hell you are there. Psalm 139:8) - indicating that God is everywhere at once, which is not a hard thing for God to do, because He is God and has ALL POWER. Is anything too hard for the Lord? (Genesis 18:14)