We used our raincover three times in October; twice for light rain and once for a downpour.
But I bought one ultimately based on our experience in 2002 (pre-kids for us). On our MGM day of that trip, it very suddenly DOWNPOURED for about 1.5 hours straight. I kept seeing parents trying to keep their kids dry and it looked awful. Then, the next morning at the bus stop, there were some people by us with a big double stroller with the real cushioned seats and their kids were already crying and unhappy. The seats of the stroller were so wet from the rainfall the day before, that the poor kids little butts were soaked before even leaving the resort the next day. I cant imagine how much fun that day was for them. They were trying to get the kids to sit on some rain ponchos instead of the seats.
I figured it was a50/50 chance of us needing it, but I did not want a wet and musty stroller if it did happen to rain.
IDK about two single covers, try it out with the one you have and see how it looks? The only thing I could see as apotential problem would be right behind the front seat, where the back seats footrest is, would have a gap of coverage between the two raincovers. So I would think any runoff rain would end up dumping off the covers into that area, which depending on your stroller, likely leads right into the basket? So you might get a real wet basket.