Connecting iPad & iPhone Simultaneously to a Multi device adapter

There are multiple reasons why you'll not be able to achieve what you are perhaps attempting. You have encountered fundamental limitations of the USB architecture that apply not only to Apple, but to the majority of computing systems. While USB provides a method by which to connect various types (or classes) of USB device, not all can be interconnected or simultaneously connected.

Your USB multi-port Hub is designed to be connected to a single host computer - which will internally incorporate a USB Controller and USB Root Hub. The host computer (of which there can only be one) can access multiple USB storage devices in a one-to-many topology. In general, USB doesn't support the many-to-many connection topology that you are attempting to create; you can't simultaneously connect multiple host devices to multiple storage devices.

Your hub also includes a USB connection for your Power Adapter, this dedicated port being intended to provide power for the host computer, the USB Hub and any USB peripheral devices that are also connected. Very often, the host computer cannot alone source sufficient power to run all of the connected USB devices.

Next, your iPhone does not include a USB Host Controller or USB Root Hub with which to connect and access USB storage devices - but the Lightning to USB Adapter that you connect to the iPhone does include these essential components; this Adapter allows the iPhone to connect with and access USB storage devices. This Adapter also supports connection of USB HID (Human Interface) Devices) such as an external USB keyboard.

Next, your iPad and iPhone (and most computers) do not expose their internal storage in the same manner as a USB memory stick or other USB storage devices - and in general, cannot be used in the same way.

As for your immediate aims in easily transferring data between your iPad and iPhone in absence of a network connection, consider using AirDrop - this providing a simple mechanism to wirelessly transfer files from one device to another. This works both ways, the transfer being initiated from the source device.

How to use AirDrop on your iPhone or iPad - Apple Support

To use AirDrop, simply enable WiFi and Bluetooth.

For AirDrop to work your iPad does not necessarily need to be connected to a WiFi network - or Bluetooth paired with other devices. AirDrop automagically detects other devices that can receive files using AirDrop.

Assuming that both your iPad and iPhone are signed-in using the same Apple Account / AppleID, the process is fully automatic.

there are manual switches like these Amazon.com : usb manual switch

but usb do not really support an accessory having a connection to 2 masters at the same time it's more of a peer to peer setup

haven't tested any with ios devices, but it's a physical switch like your average wall power switch so it's functionality is like if you manually disconnected the cable from 1 ios device and then connected it to the other.

You're very welcome.

I do suggest that you explore AirDrop; it provides a very simple method to wirelessly transfer files between devices.

don't really use airdrop but, I am under the impression that it form a direct peer to peer wifi connection with the 2 devices rather than rely on both of them being connected to the same wifi network