In ASIAIR app, open the ASIAIR Settings window (the one with the WiFI icon).
Scroll down until you see "Switch Device." Tap on that.
Assuming the ASIAIRs are connected using station mode, or using Ethernet cables, you should now see all the ASIAIR devices on your network. Choose the one you want to connect the iPad to. If it does not show up, connect using the IP of the ASIAIR you want.
If you are connected using the ASIAIR WiFi hotspot. You need to go to the iPad WiFi settings and choose the correct hotspot.
The ASIAIR does not choose which iPad it wants to connect to. Your iPad chooses which ASIAIR it connects to.
Each ASIAIR allows up to two iPads to concurrently connect to it. ( I don't know if the crippled ASIAIR mini has limited that number too. As usual, technical documentation from ZWO is missing.)
Chen