So this is what the QW 787 aircraft programmer said:
I think your user may have a misunderstanding regarding the displays.
See the chart below.
What your user is referring to as the CDU is the Lower Display. The Lower display is split between the Capt, and FO side. The Left side is the Capt Side, the right side is the FO Side.
Each side has its own control panel. If you look at the red rectangle you’ll see the ‘CHKL’ button. When the CHKL button is selected on the right side of the lower display, the CDU is replaced with the ECL.
When the lower display is not in full map mode, anything on the right side of the lower display is the FO’s, anything on the left side is the Capts.
The behavior your user is describing is correct and is inline with how we said it would work in my mail to you on 4/25. I’m repeating the described sequence below.
1) Check if the FO's checklist is already open, if so just send the inputs wherever it is open.
2) If it’s not open, but the Eicas screen is on the FO's side, it opens the checklist on the FO's side of the Lower display.
3) If the Eicas is not on the FO side, then it will open the checklist on the left side of the FO’s Inboard display.
So if the checklist is already open in another position (either the right side of the First Officers inboard, or the right side of the Lower display the inputs will still be received).