I thought like you about a SCP limitation, so I used a DS4W build which uses the SCP drivers.
Since I installed IM first, in my steps above, the SCP drivers were installed by IM (not by the DS4W built-in installer).
Then I used DS4W with SCP. And it worked with both pads at same time...
It's a good thing the future 1.7 will use the ViGEm drivers, they will have lower latency with BT (so I heard).
Nice idea, I will try the beta 1.7 then, and see if I got the same issue.
For fun, I did try to run both DS4W and IM at the same time
When using DS4W with SCP, it froze IM. Both apps seemed to struggle to get access to the underlying hardware.
When using DS4W with ViGEm, the pads were constantly switching from one app to the other, like, IM manages to access the pads when all of the sudden DS4W steals them and it goes back and forth like this forever.