Thank you for both your replies.

@Pmb:
what would be predicted for special relativity?

I'm still a bit confused. The original twin paradox is explained away with the asymmetry (one twin accelerates, so not in an inertial frame). As you said, this one is symmetrical and both are in inertial frames throughout. Shouldn't the time dilation equation be applied for both of them? And if so wouldn't they measure the other to be younger, seeing they have been travelling a constant relativistic 2v relative to each other their whole lives?