This isn’t directly related to EVE-O Preview however until the option to one-key cycle through your clients you can accomplish this with the Windows app AutoHotkey (https://www.autohotkey.com/).
Here’s my script for switching between two clients. As you can see from the comment I use a button on my mouse to emulate ALT-SHIFT-T which cycles between ALT-SHIFT-I and ALT-SHIFT-O which are my EVE-O Preview keybinds:
#NoEnv ; Recommended for performance and compatibility with future AutoHotkey releases.
; #Warn ; Enable warnings to assist with detecting common errors.
SendMode Input ; Recommended for new scripts due to its superior speed and reliability.
SetWorkingDir %A_ScriptDir% ; Ensures a consistent starting directory.
; Pressing ALT-SHIFT-T will toggle between the two main Eve windows, ALT-SHIFT-I and ALT-SHIFT-O
!+t::
toggle := !toggle
if toggle
Send !+{I}
else
Send !+{O}
return