Escallation takes time. In WW 2 for example, the various combatants didn't go bombing cities into ruin wholesale from day one. During the Polish campaign only Warsaw could be said to get something like a nuke treatment where the Luftwaffe bombed it mercilessly for weeks. Other Polish cities didn't get this sort of concentrated treatment.
During the BoB, the Germans initially avoided attacking cities fearing retaliation and stuck to military targets. When they became desperate, they switched to terror bombing and got the same in return. If the two were tossing nukes, the Germans tossed one, the British fired two or three in return. Things went up (or down depending on your POV) from there. There was no single day of 'we'll fire everything we have' and getting the same in return. Neither side wanted that sort of destruction.
Germany refrained from using gas as did the Western Allies during that war even though both sides had massive quantities of it. Same idea.
So, even two nuclear armed opponents may refrain from tossing nukes indiscriminately more out of fear of retaliation than anything else. It's the crazies like those ruling Iran or N. Korea that you have to worry about. If Putin really is crazy, then he could be dangerous, but we really don't know at the moment.