Ramakrishna says that the Jivakoti can ascend to the Divine but may not descend, while the Ishwarakoti can ascend and descend at will. Sri Aurobindo says that going by this definition ‘Ishwarakoti are those who get to the integral Reality and can therefore combine the Ascent with the Descent and contain the “two ends” of existence in their single being.🤨’ -[Conti]

Does that mean that Ishwarakotis are having a supramental experience as they are seeing the ‘two ends of existence’ actually become one?

By definition, it seems so. Ramakrishna often alluded in his talks that Brahman and his Shakti are One and the world is their own manifestation and play.

Could not decide what this was because the idea of Ascent and Descent is not so well-formulated in other yogas but I did find in Ramakrishna’s talks he always talked of Oneness and considered the world to be real and a play of Shakti.

I always understood this statement from Sri Ramakrishna (of course before contact with Sri Aurobindo) to be describing souls that have arrived at a state of inner freedom and yet choose to return back to earth for doing the Divine Work. In this sense, the word ascent means the soul’s ascension into a state of Brahmanirvana. Descent refers to the soul returning back from any of the higher planes abandoning the complete dissolution. Naturally this is only possible if somewhere the soul intuitively senses that Reality is integral and is as much here (though hidden) even as it is above and beyond where it is self revealed without covers. This intuition one finds in the Vedas, Upanishads, the Gita and other mystics as well. There are souls that are not only aware of it but choose to come down to help. The Mother has called them heroic souls. Savitri herself is described as one of them. 

‘One  had  returned from the transcendent planes

And bore anew the load of mortal breath,
Who  had striven of old with our darkness and our pain;
She took again her divine unfinished task:
Survivor of death and the aeonic years,
Once more with her fathomless heart she fronted Time.’
The Mother was ‘born free’, a perfect Jivanmukta as Sri Aurobindo saw. But all this has nothing to do with transformation which is not about the soul’s ascension and return but of Nature. 
Integral Reality can be sensed on the Intuitive plane if one has taken that route. One need not actually climb to the Supermind or bring it down to know it. Sri Aurobindo himself knew about it much before he entered the Supermind. This is the sense that comes if we read through all that Sri Aurobindo wrote on this subject as cited below.

‘I don’t think I have written, but I said once that souls which have passed into Nirvana may (not “must”) return to complete the larger upward curve. I have written somewhere, I think, that for this Yoga (it might also be added, in the natural complete order of the manifestation) the experience of Nirvana can only be a stage or passage to the complete realisation. I have said also that there are many doors by which one can pass into the realisation of the Absolute (Parabrahman) and Nirvana is one of them, but by no means the only one. You may remember Ramakrishna’s saying that the Jivakoti can ascend the stairs, but not return, while the  Ishwarakoti can ascend and descend at will. If that is so, the Jivakoti might be those who describe only the curve from Matter through Mind into the silent Brahman and the  Ishwarakoti those who get to the integral Reality and can therefore combine the Ascent with the Descent and contain the “two ends” of existence in their single being.’

‘The language of the Gita in many matters seems sometimes contradictory because it admits two apparently opposite truths and tries to reconcile them. It admits the ideal of departure from sansara into the Brahman as one possibility; also it affirms the possibility of living free in the Divine (in Me, it says) and acting in the world as the Jivanmukta. It is this latter kind of salvation on which it lays the greatest emphasis. So Ramakrishna put the “divine souls” ( Ishwarakoti) who can descend the ladder as well as ascend it higher than the ordinary Jivas (Jivakoti) who, once having ascended, have not the strength to descend again for divine work. The full truth lies in the supramental consciousness and the power to work from there on life and matter.’

‘As to the impossibility of return [to the earth], that is a knotty question. A divine being can always return—as Ramakrishna said, the  Ishwarakoti can at will ascend or descend the stair between Birth and Immortality. For the others, it is probable that they may rest for a relative infinity of time, śāśvatīḥ samāḥ, if that is the will in them, but a return cannot be barred out unless they have reached their highest possible status.’

‘Ramakrishna himself never thought of transformation or tried for it. All he wanted was bhakti for the Mother and along with that he received whatever knowledge she gave him and did whatever she made him do. He was intuitive and psychic from the beginning and only became more and more so as he went on. There was no need in him for the transformation which we seek; for although he spoke of the divine man ( Ishwarakoti) coming down the stairs as well as ascending, he had not the idea of a new consciousness and a new race and the divine manifestation in the earth-nature.’

Affectionately,

Alok Da