Here is a little thought experiment: there is one A380 and two B777 flying with all the engines having the same failure rate. Now is there same probability that one of the four engines in the A380 fails as that one of the four engines in the two B777 fails?
The answer to that is dependent on the definition of "failure rate".
For an airframe as a whole, an accident rate is defined as "per 1,000 hrs flown"
For an engine, the defined failure rate may appear similar, and yet slightly different; i.e. "per 1,000 hrs" but this is applied per engine
Thus a B777 might fly a total of 21,000 hrs, and the engine clock on each engine registers 21,000 hrs, but that engine type will register 42,000 hrs trouble-free service.
If that same B777 then suffers a single engine failure and subsequently crashes, the airframe will have a failure rate of 1 in 21,000 hrs, but the engine type will have a failure rate of 1 in 42,000 hrs (per engine), corresponding to the fact that the second engine was in perfect working order up to the moment the B777 hit the ground
To answer your question; yes, in theory one of your B777s would have an engine failure at exactly the same time the A380.
The whole essence of the 4-engine vs 2-engine argument, is what happens next? The A380 (on 3 engines) has the luxury of considering it's options, and may even continue on to it's original destination. The B777 struggling along on 1 engine is most likely looking for the first opportunity to reach terra firma. A further consideration is what happens if the unthinkable happens - a second engine failure. For the A380, this would be quite bad news, and much more so if the double engine failure was on the same side, resulting in asymmetric thrust. Meanwhile, on the B777 glider - I'll let you do the math.(*)
(*) Ok, so it is also possible that the second
engine failure on the B777, happens to the second
B777, giving us two single-engine B777s. But that's another can of spaghetti.
I was going to add a witty sign-off, but all the best ones have already been taken.