Are "Diagonal" Attacks More Deadly in Diplomacy?
It's definitely the case that fighting a two-front war is more than twice as hard as fighting on one front.  Besides the obvious reason, that you need twice as many units, there's the support-cut effect too.  Most empires, especially early, aren't 3 provinces wide, so even an unsupported attack on one side can strip a unit on the other side of the support it needed to hold.

One place this is borne out is in the aggregate results of Russia.  Russia has the highest solo rate of any power, and the second highest elimination rate.  If Russia can't turn their initial two-front (North and South) exposure into a one-front war, they're done for in a blink.