> is it NetBSD problem or hardware impossibility.
> i tried it configure same irq on buildin coms and extra 2-port ISA com
> adapter. it is detected at boot fine but then it simply doesn't work (no
> traffic).

I've tested the same case with 2 ISA ethernet cards : a ne2000 at irq 10 and
a 3c509 PnP taking the same irq. The two cards were detected at boot, the
3c509 worked well but the ne2000 never want...
I've moved a jumper to irq 5 and recompiled a kernel with new settings then
it worked.

> is ISA hardware incapable of interrupt sharing or just NetBSD can't do it?

I don't think ISA have interrupt sharing capability but NetBSD seem not to
be disturbed.