Does the 2 night cross over began's on Super girl and what would bring team arrow to Kara's Earth. Who would like to see to the big bad for this cross over

Since Supergirl will be paired with Arrow on Monday for the Mega-Crossover, it has to start with Supergirl's world. I think Supergirl, Mon-El, Martian Manhunter, and Winn may be the ones from
Supergirl to crossover. I don't know how Alex will take to the idea of crossing over to another dimension.

I don't see why Alex would have a problem going to another Earth. She had no hesitation going to another planet using the transmatter portal in S2E9 "Supergirl Lives". Alex is a scientist and in her own way a nerd so she probably gets the whole multiverse thing like Winn did in S1E18 "worlds finest". Kara trusts Barry, Oliver, Cisco and everyone else on Earth-1 so that will be enough for Alex to trust them too. She may be mad at Oliver for being a jerk to Kara but since Kara ultimately worked things out with him Alex will be fine with him.

Since Supergirl will be paired with Arrow on Monday for the Mega-Crossover, it has to start with Supergirl's world. I think Supergirl, Mon-El, Martian Manhunter, and Winn may be the ones from
Supergirl to crossover. I don't know how Alex will take to the idea of crossing over to another dimension.

At the end of first 4 night cross over they did knowledge that On Earth 1 Kara help established there verison of DEO.

So what if head of their DEO and same of personally came's to Earth 38 and observed DEO in action and how they capture alien's that are breaking the law and help protect those alien's who are peacefully law abiding citizen's in National City.

I could see that perhap's Alex & OLiver who can be both strong headed same time's clash but at the end of it the have gained mutually respect for one other.
It think it would kind of cool of they do other cross over where Alex is on Earth 1. She there's to help trained Earth's 1 DEO. Before she leave's to go back to Earth 38. Same thing happen's that require's her to go to Star City to Help team Arrow.