> is there any support to dual monitor setup?

no.

> Can I rotate monitor in plan 9?

no.

> How does support of video cards looks like in general?

poor.

see /sys/src/9/pc/vga*.c files. most of these are very old. the only
realiable way to use a video card on 32- and 64-bit intel platforms is
to use VESA.

mischief