Sorry, cannot say. But you should not rely on MS task manager. Mine would just not show any workload on my second gpu no matter how hard it rendered. That said MSI Afterburner shows it all.
For those of you with ignominious multi GPU (I run GTX 970 with RTX 2070): I found that most of the time rendering my 2070 was idle waiting for the 970 to finish its bucket before starting the next bucket itself. For me the trick was to NOT setting the bucket size as large as possible but really small. With 32x32 my 2070 almost has no waiting idle time anymore. And despite all info out there in the web: even with only one card running smaller bucket sizes gave me performance plus of maybe 7-10% (on both the 970 and 2070).