The communist "failure" in Greece had more to do with the the British and then US and the large supply of arms and materials to the
Kingdom of Greece.
In OTL Stalin was wary of pressing into Greece. He had agreed with Churchill in the percentages agreement and at Yalta over the situation in Greece. You either have to change those agreements I'm not sure how you would get Churchill to agree nor FDR later to more Soviet sphere of influence in Greece. Stalin also did not think that the USSR was ready to challenge the West in 1945-1946. Stalin was more concerned with consolidating the later Warsaw sphere of influence.
Tito is also going to have too behave differently to prevent the "Split". He already had the US-UK looking at him badly due to the attacks on the UK/US supply the tail end of WW2, Tito wanting territories from other Soviet sphere states, and not taking aid from the US in the Marshall plan. This is almost a Stalin isn't Stalin and Tito isn't Tito situation.
Pouring in of Aid from the USSR to Greece is going to have the West look at Stalin in a way that they can not trust Stalin in anyway after a reversal of previous agreements and discussions. You are going to get suggested reversals by the West on previous agreements that Stalin might not agree and push things into the Cold War faster and make things hot. Stalin for all the crazy things he did isn't stupid. He was happy for Greece to fall out of the Soviet sphere for things in his mind are more important.
Pouring of aid, men, and materials from Yugoslavia is going to get Washington and London ringing Moscow going WTF? Along with a please rein in Tito or we will. If the waves of assassins can't get Tito then the US/UK are going to intervene. Tito being scared of Greece invasions of Albania/Yugoslavia will be replaced with scared notions of the US/UK coming instead. I'm not sure Tito would want to press this. He has too much to loose. Yugoslavia needed the aid from the West and was one of the concessions for Yugoslavia to stop aiding the Communists in Greece. If Yugoslavia is full bore aiding the Greek Communists then the aid from the US isn't coming Tito knows this.