I've assigned hotkeys for the commands "Go BACK" and "Go FORWARD" to the keys "Ctrl + Left Arrow" and "Ctrl + Right Arrow", respectively. Executing these hotkeys will browse back/forward, but the resulting folder will be opened on the right pane of a dual vertical setup (having started from a single pane). My Back/Forward toolbar buttons, and even my mouse gestures, which use exactly the same command work perfectly.
What could I be doing wrong?
By default the Ctrl + Left/Right hotkeys move you to the previous/next tab. Maybe you've confused Opus by having two commands on the same hotkey.
Go to Settings->Customize->Keys to get a list of hotkeys. Any conflicts will be shown in red. You may also see that Alt + Left/Right arrow are defined as Back/Forward by default (depends if you've edited your toolbars/hotkeys, of course).
Just happened upon this thread, although it doesn't appear to have the solution.
I just checked my hotkey settings, and there aren't any red entries or duplicated key combos. I've reassigned prev/next tab to "Num 1" and "Num 2" to mimic Opera. "Ctrl + Up Arrow" works fine for "Go UP", so the problem isn't the Ctrl key.
edit: I just reassigned back/forward to different hotkeys, tried pushing Ctrl+Back/Forward, and nothing happened... I guess this means there was no conflict.
edit again: If I try to use the Ctrl+Back/Forward hotkeys in the right pane of a dual mode lister, nothing happens. It only works with the left pane focused, and always opens in the right one.
Which locale are you using? I wonder if it's specific to e.g. French?
I'm using the English version.
I just tried disabling the hotkeys in Opus and creating something equivalent with Litestep. The idea was to feed the "go BACK/FORWARD" commands to dopusrt.exe. Opus received the commands just fine... but the problem persists.
I've also found that the problem has nothing to do with using Ctrl+Back/Forward as a hotkey combo. I changed Ctrl+Up to Ctrl+Back for my "Go UP" hotkey, and it worked fine.
edit: Well... it seems that clicking on the file display border Back/Forward buttons while holding Ctrl does the same thing. Is there some option for such a modifier key? Don't think I've seen it anywhere.
I've filed a bug/feature report for you.
i never find the problem... 
There's actually an undocumented parameter you can use in conjunction with Go BACK/FORWARD to prevent this from happening. Add IGNORESHIFT to the command and it will ignore the state of any qualifier keys (shift/ctrl/alt). Eg,
Go BACK IGNORESHIFT
Thanks Jon, with this parameter dual mode do not open !!!