I think the Soviets would have a far harder time accepting Ukraine's loss (which had a large portion of Russian industry and population) than Siberia (much tinier in population and industry though high in resources), and Ukraine doesn't have a convenient power wanting to set up a buffer state. Maybe if the Germans win WW1, but none of the victorious powers really have enough interest in the region in the region to back it long term, other than the interest of 'Stick it to the Communists', which would fade as Communism became seen as more legitimate.
Britain and France would be willing to support the Ukraine a little, as long as they don't have to get involved themselves they're fine.
The big support though I would see would come from Poland. The Poles were rather strong post-war and they would be very interested in a weaker, split Russia.
Yep, Russia would be less willing and more annoyed at losing Ukraine whilst Siberia would be nothing...however the same is true for other nations. A independent Siberia no matter its legitmacy and connection to old Russia is rather unimportant. The Ukraine though, a nation in Eastern Europe...now that is important,.