Put a file (app_config.xml) in your BU project folder that contains this:

<app_config>
<app>
<name>lineB-sgminer-4.1.153-v2</name>
<max_concurrent>2</max_concurrent>
<gpu_versions>
<gpu_usage>1</gpu_usage>
<cpu_usage>1</cpu_usage>
</gpu_versions>
</app>
</app_config>

Then go into BOINC and under the ADVANCED tab tell it to read config files.

The BOINC scheduler will reserve 1 thread for your GPU.