Multiple WU's on GPU

[TouchuvGrey]

My two Radeon RX580's are happily running 2 WU's each using

<app_config>
<app>
<name>milkyway</name>
<gpu_versions>
<gpu_usage>0.5</gpu_usage>
<cpu_usage>.05</cpu_usage>
</gpu_versions>
</app>
</app_config>

in Program Data~ BOINC~projects~milkyway.csi.rpi.edu_milkyway

but i cannot get my R9 280X or my RX5500XT to run more than one WU.

    Any thoughts ?