Long story short: Theoretically possible, logistically difficult and completely nuts.
I mean besides pissing off the west, what does this plan acchieve for the sovs? It swaps one running sore of an insurgency for two.
Likewise, what's in it for India? Is exchange of a few border issues and the occasional skirmish for open war followed by a lengthy insurgency (and that's if things go to plan...) really worth while?
To be fair - Afghanistan looks like it will prove a relatively short victorious war after all.
I think it would inevitably spiral into a third world war. Just as in WWII with chemical weapons the Balance of Terror shall keep things from escalating into unrestricted WMD use so it comes down to the military balance. Given the USSR having a superior production capability in terms of weapons in the short term they shall probably take Madrid before their enemies can mobilize for a proper war economy.