It has long been misunderstood that ordination of bhikkhuni must be done by both sanghas. For this reason bhikkhuni ordination was seen as not possible as there were no bhikkhunis in Thailand through 700 years of history.
Then the bhikkhu sangha continue with the upasampada. I would like to emphasise that upasampada is done only in bhikkhu sangha not in bhikkhuni sangha and then again in bhikkhu sangha as it is done in Sri Lanka at present.
I would like to quote from Cullavagga,Vinaya Pitaka no.574
“Anujanami bhikkhave ekato upasampannaya bhikkhunisanghe visuddhaya bhikkhusanghe upasampadetunti”
Translated in Mahamakut version of the Vinaya as
“The Buddha said “ O bhikkhus I allow the candidate ordained in one sangha, she is purified in the bhikkhuni sangha and receive upasampada in the bhikkhu sangha.”
In case the candidate received upasampada both in bhikkhuni sangha and then bhikkhu sangha, the first upasampada done by bhikkhuni sangha is invalid, as it is not prescribed in the Vinaya. As long as the candidate receive upasampada in the bhikkhu sangha her ordination is complete.
With this understanding one will have a clear view that the upasampada is not done in both sanghas, rather the candidate seek bhikkhuni sangha to help her with the purification of the antarayikadhamma only, the upasampada for bhikkhuni is still to be done by the bhikkhu sangha.