Looks like a case of bad luck to me so far. 24 disks, two fail in the same week? Unfortunate, but it can happen. Especially if the disks in question are 7.2K RPM drives not intended for 24x7 operation. I don't see any 500GB drives on the QuickSpecs sheets for the current LeftHand products, but I do know HP did sell 500GB 7.2K SATA drives as I have a bunch of those in an MSA1500.
If those are indeed the same drives I have, I'm not at all surprised. Those came from an earlier model of SATA drives that weren't as reliable as their SAS/SCSI counterparts, so have shown a higher failure rate here. After the first year, though, the bad apples worked their way out and I haven't had any failures since. But I was going through one a month there for a while. 5.5TB LUNs are the largest I'd suggest running with those drives, as the rebuild time (as you're probably learning right now) takes a VERY long time.
If they're really 450GB 15K SAS drives, that's much less probable but still within the realm of possibility. Sometimes these things happen.
More broadly, I know HP likes to sell LeftHand nodes in pairs. Presuming both of these units were obtained at the same time, the likelihood that all the drives are from similar batches is pretty high. As Chris S pointed out, bad batches happen. Since 5.5TB RAID5 sets can take a week to rebuild (don't have a LeftHand SAN to play with, but I know for MSA-based arrays it takes a week), and during that time the drives are under a much higher load than normal, it can cause failure cascades. However, you said "last week" which suggests they've been rebuilding for a while now and are just looking for the suspicious alignment of stars that caused the failures in the first place. If they've survived the rebuild process, it's less likely you have a seriously bad batch, maybe only slightly bad. But do keep an eye on failure rates.
2 in a week is a data-point, not a trend. Unfortunately.