You need two alpha particles hitting the same rank of memory for failure to happen. Although super rare, even then it is still correctable. You need three before it is silent data corruption. Silent corruption is what you get with non ECC with even a single flip.
Where are you getting this from? My understanding is that these errors are predominantly caused by secondary particles from cosmic rays hitting individual memory cells, and I've never heard something so precise as "you need two alpha particles". Aren't the capacitances in modern DRAM chips extremely small?
The structure of the ECC is at the rank level. This allows for correcting single bit flips in ranks and detecting double bit flip in ranks. So when you grab a cache line each 64bit is corrected and verified.
