At a total guess, code completion is somehow mixing the compilers?
Did you think it's CC's problem?
An error box will pop-up titled "cc1plus.exe - System Error" that states the program can't start because libgmp-10.dll is missing from your computer
This happens when the second .cbp is loading?
Edit:which version of CB did you use?
I do not have a Win64 system/Win64 compiler, so I do not have the test environment.

all above was from my guess.