Calling it a single conflict is bit of hair splitting, japan had almost nothing to do with germany, especially after war started. Those were 2 separate conflicts happening at the same time.
Part of what combined the two conflicts, aside from Allied questions about resource allocations and priorities, was the supply lines of the British Empire. There was a continuous series of supply lines running all the way from England to Burma, through the Mediterranean and Suez Canal and down through South Asia. The Suez Canal (under threat from the Italians and Germans) was part of the exact same supply line as the Burma Road (under threat from the Japanese).