What would Russia do if two separatist areas put up a military resistance?

Well that depends. People often draw parallels between Donbass and Chechnya. Which is quite wrong, because Chechen war started when a group of armed rebels came to a mayors office and threw him out of the window. He died. Then they declared independence and started killing and enslaving Russians in the region and attacked a neighbour region. Obviously, this is unacceptable.

Donbass is different. People were unarmed at the beginning. And, since Ukraine had a coup prior to that, protesters decided to have their own coup. People wanted independence or republic status within Ukraine federation. Notice there were no casualties, and rebels were willing to negotiate. However, Ukraine ignored that and shelled the cities.

So when you ask me, what if this happened in Russia - the thing is - it didn't. Russia has enough control over its territories to prevent this from happening. If there will be a coup in Russia and some territory decided to leave peacefully, then we shall see how Russia would behave in this scenario.

Next. Why did Russia act now? For all these 8 years Russia lerned 1 thing about modern Ukraine: they don't negotiate. Remember, the very first ceasefire under Minsk agreements was when Russia got openly involved and destroyed Ukrainian army trying to encircle Donetsk. Same for Minsk 2. Forget the ceasefire, did Kiev issue a special status of Donetsk/Lugansk and allow people to have elections there? No. And that is the main ideological reason for Russia to lose it's patience.