2 waves at right angles applied to a 2d array of cubes (Dupliverts)

For an animation that looks like boxes floating on water, such that there is a sine wave spreading in x axis and a cosine wave spreading in y axis that plays at the same time.

If I apply 2 wave modifiers to a plane and cube (using dupliverts), I get a wave starting and finishing then the next wave starting and finishing.

wave.modifiers

How can I make them both play at the same time ?

Found the effect at ChrisP’s Animation Node tutorial.

How can I replicate the same effect without using Animation Nodes ?