Is it possible to summon Durnehviir and shift into vampire lord form and summon a gargoyle and have both at the same time? Also possible to have Durnehviir and conjure boneman or other summon spell?

I just started playing with conjuration spells and just found out that normally only 1 creature can be summoned at one time and wasn't sure it applied to Durnehviir. I've tried before but he normally takes off into the sky chasing after another dragon and it's hard for me to keep tabs on what's going on.

Possibly almost certainly with dual souls

Xbox loads too slow for my taste and was hoping someone would know the answer. I tested and tried without dual souls perk, and apparently Durnehviir does take up a summon slot no matter if in VL or mortal form. This includes summoning Arvak.

Doesn't Durnehviir count as a shout?

Duurnevihr (?) takes a summon slot, yeah. I've tested Twin Souls as a VL, and it doesn't seem to work. You can transform just fine though, and the dragon will aid you until his timer runs out, or you summon something else. By the way, Duurnevihr also summons a Boneman, Mistman and Wrathman of his own, so it's not like you'd be outnumbered or anything.