If you place two chained command blocks with the same command: "execute at @p run summon arrow ~ ~3 ~

" and activate the first one (which activates the second one at the same time), 2 arrows will be spawned above you and you will get about 9 hearts damage. If you do the same with only one command block, you will get the same damage. So the damage of arrows hitting you at the same time is not added. You will just get the damage of one Arrow.