Well, at maturity, giant arborvitae can reach 10-16 feet width so the gaps won't be...well, gaps.
But till then, if you don't mix evergreens in the second row, during the winter the gap will be visible.
For the second row you can try wichita blue juniper or other round shaped evergreen to add contrast to the tall shaped arborvitae. Try though no to mix more than two or three colors.

Tall conifers of another color and texture also would look good. Picea omorika (many pendulous forms), Picea glauca 'Pendula', Picea engelmannii 'Bush's Lace', Abies concolor 'Conica' to name a few. How wide would the second row be?

tj

Can you actually find any of those cultivars up there?

"Can you actually find any of those cultivars up there?"

For all the ones I mentioned and several of the ones Dax mentioned, yes, they are at better nurseries around here.

tj

Does an emerald green and gold arborvitae go well together alternating each? Thank you

If you want color that's pretty wild I would think. I believe three in a row of each would be more pleasing. It might still be overdone.

Dax

wonder what this new post has to do with a 7 year old post about green giants?

ken