There are 2 questions here.
Is jiva an upadhi of Nirakara Brahman or saguna Brahman?
Partial answer: One cannot even think of jiva or Isvara when thinkng of Nirakara Brahman. They become undefined since Nirakara Brahman is all that exists. A fuller reply will be given relative to the second question.
Do Advatins consider two types of Purusha?
Let me first post two relevant verses from Pancadasi which explain Isvara and His relationship with Jiva..
Sruti says that this (pure universal) consciousness reflected in Maya
is Isvara which controls Maya as well. The great Isvara is the inner
ruler, omniscient, and cause of the universe.
Vide Svetasvatara Up. 4.9-10, Mandukya Up 6
Pancadasi of Sri Vidyaranya Swami VI.157 translated by Swami Swahananda
The Sruti, in the passage beginning with 'the consciousness in the
deep sleep' and ending in 'He is the Lord of all' describes this
'sheath of bliss' as the Isvara.
Pancadasi of Sri Vidyaranya Swami VI.158 translated by Swami Swahananda
Vide Mandukya Up. 5-6, Brhadaranyaka Up. 4.4.22.
Here Isvara is the consciousness reflected in the totality of
bliss-sheaths. It has been stated above that Isvara is the reflection
of consciousness in the Vasana, and as the bliss sheath is nothing
less than such a reflection, it is called Isvara. This, however, is a
concession to those who cannot comprehend the higher conception in the
Here the author clearly says that this 'sheath of bliss' is Isvara.
Much dust of controversy, as over the previous sloka, has been raised
over this statement. The point, however, is this: A Jiva is a
reflected consciousness individualised, and Isvara is the reflected
consciousness of the totality. Now can there be totality without
individuals? So Isvara must of necessity, include Jivas. In each drop
of Ganges water the Ganges abides, and yet all the drops severally do
not constitute the Ganges, which over and all of them is a unity.
Each individual soul, Jiva, has three states of consciousness, the
waking (when the Jiva is called the Visva), the dreaming (when he is
called Taijasa) and the deep sleep (when he is known as Prajna).
Isvara, the collective consciousness, has also three corresponding
states - the Vaisvanara, the Hiranyagarbha, and the Isvara. Isvara
being omnipotent, each of these three states includes within itself
the corresponding states of the individual souls. Just as the three
states of a Jiva do not make him three, even so the three states of
Isvara do not make up a triple personality. Just as Prajna, being the
causal state of a Jiva, includes the other two states of his and is
the guide and controller of them; so the third state of Isvara, being
the causal one, is the source, the indwelling Spirit, controller and
guide of the other two and through them of the entire universe.
The 'sheath of bliss' is the causal state of a Jiva. But is not the
universal cause Isvara, as well in a deeper sense? A spot in India is
also a spot in the continent of Asia, in the globe, as well as in the
universe.There is a great layer of ignorance when we attribute it only
to India and not the universe. The individual 'sheath of bliss' is
only a portion of the universal 'sheath of bliss' and when the
individuality and all the Vrittis vanish, how is one to distinguish
between the individual and the universal?
The fact of returning to the individuality in the later waking state is
no bar to its previous merging in the universal - diversification
starting on the return journey when one comes down to
'I'-consciousness, left long behind.
Commentary by Swami Swahananda
The Isvara may be thought of as the sum total of all purushas while Jiva is the individual component of Isvara.