If by any chance, they can form an alliance and come to power they would start fighting like cats and dogs. There is no doubt about it.
Non-violence is the greatest Dharma; So too is all righteous violence.
'Idhuvum Kadandhu Pogum "
Even this challenging situation would ease
All the above are bitter facts and all the individuals have reached the position by accident and due to the prevailing political scenario in the country at that time. So, anything can happen in Indian politics.
If there is a political crossword going on in the opposition camp, I am very sure that things are not very different in the NDA camp too. Equations, permutations, and combinations can change any time and we will have to wait.
I personally feel that it is too early to discuss anything related to the 2019 elections as of now. Let us be patient. Hope our members will understand.
'Beware of false knowledge; it is more dangerous than ignorance'. - George Bernard Shaw.
As far as Indian political history and PM is concerned, we know exactly how long Mr Deve Gowda, Mr Guijral and Mr Chandrashekhar had held their PM chair with the support of Congress party. We have seen the musical chair and political circus many a time in Indian political history and if one party Government don't come in the next General Election, be ready to have one more political Circus along with bunch of Jokers.
So the happenings may belie our expectation and guess.
As of now it is very difficult to see TMC president accepting TRS president as PM. But when the situation goes to such an extent, this or worse after this, then people may come to any compromise to exploit the best favourable situation. There will be haggling,bargaining, and many other methods to make situation in one's favour or at least make it against another.
So we need not surprise i after a well planned compromise, TMS president may accept TRS president as PM candidate. Then they will rationalise it and justify it with well crafted story sacrifice and unity. This can be true of X, Y or Z.too.