If the goal is to get a long-range connection with supported devices and 1Mbit with devices which do not support long range, you normally still want to scan on 1Mbit. The most common way for devices to establish a Bluetooth long-range connection is to connect on 1Mbit, exchange information, and if both devices support long-range they can switch to long range.
If you are in control of the devices in both ends of the link you may use advertising extensions to advertise and scan on long range as described here:
It is not possible to scan on both PHYs at the same time, but by quickly alternating between scanning on long range and 1Mbit you will still be able to connect to devices on both PHYs.
Could you refer me to the specific part of the specification you are referring to? I would like to follow it up with the BLE stack team in case there is something I am misunderstanding.
In general, I doubt will be supporting in the short term future. It's a very narrow use case which would probably demand complex changes to the radio, and you can get the same behavior by rapidly switching between the modes. A user would not notice the difference between a device switching fast between scanning modes and a device truly scanning on two PHYs.
