Hi, I wanted some advice on which alphabet to teach first with a child that is learning 2 languages at the same time. Do you start with one and wait till he has grasped that concept, then go on to the next. Or do you start both. Which seems to me to be quite confusing. 
This is very confusing and so far I didn’t see a one size fits all question.
My son is also raising in a bilingual environment and we do OPOL. I’m the one who talks to him in the minority language and as I’m the one teaching him to read and the alphabet, I’m teaching it in the language I’m talking which is English.
But as there are several words presented to him in Portuguese, our language, I read to him normally when it is the case, but not focusing on phonics, but sight reading. My hope is that if he really wants to read in Portuguese he’ll do so after he learn phonics in English by himself. The different will come most from the vowels.
But this is our environment. I have only few hours with my son in a day. I see several people form here who has the choice to stay with their kids all day and so they have time to do LR lesson in several languages in a single day, even the languages that the parents don’t talk.
So I think it’s up to you to do in the way and pace you feel comfortable with.
Good luck and tell us your updates! If you need any help it will be a pleasure to help.