I'm in the process of reading Faxian's travel journal that he compiled while travelling through India in the early fifth century. The text itself is a valuable historical resource as it is a firsthand account of India in the fifth century. Such works I think are extremely rare and as a result our understanding of Indian history is often somewhat vague and uncertain.
Anyway, what I find interesting is his reports that in some regions both the Mahayana and Hinayana were training together.
《高僧法顯傳》卷1：「從此東行三日復渡新頭河。兩岸皆平地。過河有國名毘荼。佛法興盛兼大小乘學。見秦道人往乃大憐愍。作是言。如何邊地人能知出家為道遠求佛法。悉供給所須。」(CBETA, T51, no. 2085, p. 859, a18-22)"From here we travelled east three days and again crossed the Xintou River. On both sides of the shore it is all flat land. When you cross the river there is a country called Pitu [Dravida?]. The Buddhadharma is thriving and flourishing. Both Mahayana and Hinayana train together. Upon seeing Chinese bhiksu the people came with great sympathy and remarked, 'How is it that someone from a borderland could know of renunciation? For the road is distant in seeking the Buddhadharma.' They completed provided everything we needed."
I also understand from reading Jan Nattier's translation and study of the Ugrapariprccha-sutra, that in the early Mahayana at least there was no stigma against Arhats and those seeking Arhatship as opposed to the few good men
who would risk much toil and suffering to achieve Buddhahood and start the sangha in some distant future.
Then somewhere along the line there was a split in the sangha.
One thing Nattier notes is that one probably reason for the split in the sangha was the difference between the position of the Arhat and the Buddha. She imagines in a sangha there would probably have been some bhiksu already named an Arhat while some young member had taken on the vocation of the Bodhisattva. This would have inevitably lead to some conflict as the Bodhisattva had already taken on a superior path and was therefore above the Arhat. As time went on the divide grew larger and larger, and before long Sravaka became a pejorative in Mahayana circles.
Nowadays more than ever before in history we have numerous schools of Buddhist thought being easily accessible and a lot of interaction never before possible. For example Zen Buddhists read the Pali Canon, Shingon priests study Tibetan Vajrayana, scholars read Pali, Sanskrit and Classical Chinese to reconstruct early Buddhism, etc... At the basic ground level too in a lot of major cities you are within walking distance of numerous temples. People can and do travel around and come into contact with a lot of different traditions.
So, is it possible that Arhats and Bodhisattvas can live together again? I suppose in a sense we're already back to that to some degree. At my university here in Japan, which is Soto Zen, we have Theravada Bhikku studying Japanese and researching Japanese Buddhism. I also know one Tibetan Bhiksuni who spent some time on retreat in Thailand at a Theravada temple. I'm aware that some intolerance on both sides might still exist, but it seems to me that Bodhisattvas and Arhats are good friends again.
The proof might be here at Soji-ji in Yokohama (Soto-shu's second HQ):
This was a gift of a monastery in Thailand to Soto-shu in Japan.
So, I guess one might say Bodhisattvas and Arhats are, at least to some degree, living together again.