No the first ever ride I believe will always be the opposite cover legend. But, if you leave the Ultra Wormhole and come back, not beating Ultra Necrozma, you can choose between Solgaleo and Lunala. After beating Ultra Necrozma, I think it will always be your game's cover legend. So Lunala on UM, Solgaleo on US.
Mmm thats the thing. First time, it was supposed to be Lunala, but im riding Solgaleo. Anyway, after beating Necroz i would be riding Solgaleo cause its my cover legend, right? If thats the case then i would see what happen. Thanks!