A twin el has a larger outlet for the spout, and a slightly smaller, restricted one for the shower head. Those differences are what allows it to work properly. ON many valves the outlet to the showerhead has a smaller port, but some of them are the same size, and the head does need the restriction to allow it to direct the water where you want. On many of those valves, both outlets are open all of the time. When they are the same size, some water will go out both. WIth the restriction, there's not enough pressure to get to the showerhead, and all of it will go out the spout (the water will rise towards the showerhead, but not far enough to come out)
Stated another way, unless there's a hard diverter in the system, on most valves, there are two outlets, and if they are the same size, water will flow to both of them. There needs to be some restriction in the one going to the showerhead so that the elevation and flow isn't enough for it to get all the way out unless you intentionally divert it. That can be done with a twin el if the valve doesn't have the restriction or a hard diverter that controls the flow to both the tub spout and the shower head directly.
It irks me when installers refuse to read and heed the manufacturer's instructions. They wouldn't say you must use a twin el unless it needed it. Most installs don't, but some do.