"sland crimea"book of Aksenov, Far East-republic, Siberin republick. If you can read russian, i can recomend you this timelines:
1/ far east republic-
http://zhurnal.lib.ru/d/dvr
2. Siberian repulick-
http://malorussia.com/sdr/index.htm
Yes, basic idea is to have easily defended and relatively thickly populated region to withstand the Red Tide. It kills "Siberian Republic" on it's tracks, as Siberia is too thinily populated and easily invaded to survive as separate political entity without massive changes with pre-Revolution POD (single remotely realistic TL with independent Siberia, "Anyone who's capable to bear arms", has post-Barbarossa industry and population relocation as it's cornerstone). So yes, Far Eastern Republic (bulwarked by Baikal fortifications against the Red wave) or Crimea. Geographically, Trans-Caucasia is possible, but this region would never receive enough Russian population to be considered "Russia" and locals are incapable of mounting coordinated defence.
If Russia get split into two countries (Imperialist Tsarist Russia and Communist Soviet Russia) is it possible that, after Vladimir Illich Lenin dies, the Soviet Russia could get split again into two other countries (Communist Stalinist Russia and Communist Trotskist Russia)?
I would say that continuing breakup of "non-communist" Russia is much more likely than split between Stalinland and Trotskyland. After all, anti-Bolshevism (not even proper anti-communism, as a lot of Whites were socialists of different stripes) was the single uniting force of Whites. Would they be able to secure substantially big area as their own, following split between "pinkos" and "reactionaries" is very likely.
Would Germany turn its attention to Russia before Western Europe if Russia was embroiled in a civil war?
It is highly unlikely that NSDAP will be allowed to come to power if there's no Red Scare. However, some kind of revanchist German regime is very likely and said regime will naturally turn it's sights eastward. It would be more correct to say that all "Russias" will court Germans to win their favours, so Germany would starts eat former Russian Empire piecemeal long before TTL's WWII. I'm not even sure WWII will happen ITTL, as Germans will have their national pride satisfied by their success in bringing Russia under their influence.