Are you trying to add a second project account to your current BOINC Manager, or do you want to run tasks for both accounts simultaneously? If the latter, then one way of doing it would be to use a virtual machine and install BOINC on the "guest" operating system. However, there will be a performance penalty for both instances of BOINC due to the overhead of running a virtual machine.

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My Detailed BOINC Stats

You can actually set the config to allow multiple clients. Suppose you have 4 cores, and you want to split it 50:50, you set each to use 2 cores. Don't know how well behaved BOINC is, but I'd imagine the OS will ensure that each single threaded job goes to it's own core... no worries about affinity [accidentally running multiple jobs on 1 core is though not unthinkable]. Trial and learn.

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Coelum Non Animum Mutant, Qui Trans Mare Currunt