How can I make this Explosion?

You don't need to make "two functions cooperate", you just write your code like you normally would. You probably are looking for something like this maybe?

function explode(pos) -- I'd make a function for it
    local sphere = Instance.new("Part")
    sphere.Shape = Enum.PartType.Ball
    sphere.CanCollide = false
    sphere.Anchored = true
    sphere.Size = Vector3.new(25, 25, 25)
    sphere.Transparency = .7
    sphere.BrickColor = BrickColor.Yellow()
    sphere.CFrame = CFrame.new(pos)
    local exp = Instance.new("Explosion")
    exp.Position = pos
    sphere.Parent = Workspace
    exp.Parent = sphere
    Game:GetService("Debris"):AddItem(sphere, 3)
end

-- Ex:
explode(Vector3.new(0, 0, 0))

Part.Touched:connect(function() -- A usage example
    explode(Part.Position)
end)