Having two main characters is not a problem, and of course you can be emotionally involved in both of them. As people say, there is plenty of literature and other media to support this.
For me, however, it tends to work the best in a story-driven context, i.e. when there is already a set arch for each character; where they begin, how they will change and where they end up. The story is what engages you, and you strap in for the ride.
In a character-driven context, this does not work in the same way, as you would now expect many different possibilites and final destinations for the characters. Basically, there is no set end-point. Here, it is the characters and what you can do with them that engages you, rather than the story. Obviously, this is the case with GGGB.
Herein lies my biggest worry with having two main characters in ORS. Eva has expressed that she wants us to care for the characters even more this game, and one can therefore reasonably assume the will drive the game. The possibilities of providing a deep enough experience with two characters, though, is much more difficult. Of course, it can still be done, it is just more difficult.
The greatest value in GGGB lay in the variation and possibilities to truly take Ashley, and to some regard even her friends, wherever you wanted to take her. Not only in terms of routes, both in appearance as well. As far as I know, no other developer had done this before.
I think this is the main issue or concern people have so far when we discuss opportunities and characters. Eva has clearly shown herself able to do something no other developer has done in terms of choices, variation and character development (both physical and pshycological). Taking a step towards making the game more story-driven/having two MC's and thereby limiting the possible variations compared to GGGB would mean that the USP is lost in a way, in relation to GGGB.
Honestly, I don't even know if what I wrote makes sense. Those are my two cents anyways. Clearly we don't really have an idea where the game is going, but I think it would be a shame to risk throwing away the qualitis that made GGGB so good.