Provided both are not being accessed at the same time. USB hubs have a max data throughput of 480mbps (megabits). To you and me thats 50MB (megabytes) per second. So if the drives both only offer a throughput of 25 each then both will work however it depends on the drives in question. ATA66 drives have a throughput of 56MB/s ATA100 drives are higher.