I think your thinking wrong, when i say drive i mean separate physical disks and not partitions of same physical disk. If each disk has 1GB total data and can perform 50MB/s read, each disk will finish job in 20 seconds. If both scan at same time it will take a total of 20 seconds to finish all, but since outadated modern AV's only scan 1 disk per time it take 40 seconds to finish.
If someone know about some AV out there which perform multiple drives scan at once, i'll appreciate that.