two separate GPUs

[Eben King <eben@gmx.us>]

I want to put one monitor (L, HDMI) on the onboard card (the one provided by
the i5) and two more (M, DVI; and R, DP) on a graphics card (GTX 970).  The
card works as well as it ever did, and its two work fine.  The onboard one
is rotated.  It is connected, but when I run "startx" it retains the
sideways text and doesn't show the X desktop.  The hardware's not disabled
in the BIOS.  How should I proceed?

I have Debian 12.9 (Bookworm), xorg something or other (not Wayland), and
XFCE 4.18.