Split cards, by definition, can only be instants or sorceries as the rules don't want half a split card on the battlefield
However, the cards are legal in Modern, and of course Commander
The token created by the additional effect of Coiling Rebirth isn't "cast," so abilities that trigger when a creature spell is cast won't trigger for the copy
Eternal Might is a balanced Commander deck that has components for setting up a drawn-out battle of attrition, or carefully engineering big turns that allow you to take out opponents in a few swings
Its companion ability requires your starting deck to contain twenty cards more than the minimum deck size (so 80 cards, basically)
It can also scry matters nicely into some fun Azorius brews