The bus is not "split in two", it is multiplexed on a per-I/O-operation basis.
i.e. only one operation can be in progress across all of the drives connected to the same bus. But that operation can use the full capabilities of the bus while it's working.
However, yes, two ATA HDs on the same cable would be slower than having them on separate controllers, if you were using them both "at the same time".
For example, when copying a file from one drive to the other - with a single ATA channel it would go like this:
while (the file hasn't been completely copied)
Start a read from the first drive
(delay while the first drive seeks, rotates, and transfers the data -
during this time the second drive can do nothing)
Write to the second drive what was just read
(delay while the second drive seeks, rotates, and transfers the data...
during this time the first drive can do nothing)
Whereas with two separate ATA channels things can be much more parallel, provided your copy program is smart enough, or if OS cacheing does readahead and writebehind effectively. The second read from the first drive can be initiated just as soon as the first one is done, and can happen overlapped in time with the first write to the second drive.
The net effect can be to double the total I/O operations per second. Theoretically. In practice things are never implemented that perfectly, but you can still expect dramatic improvements.
In a lot of cases, though, a second drive won't be used that much, and not often in time-critical ways. A typical case is a "bulk storage" or "archive" drive.
btw, the actual "transfer the data" parts are by far the shortest parts of all this.