The popcorn will likely tassel closely to the same time as sweet corn planted at the same time.[maybe a few days later] 125 day popcorn sounds like a very slow popcorn. The popcorn maturity date refers to total drying time on the stalk.

I would plant the sweet corn earlier than the popcorn a couple weeks.

Thanks for that, it's was what I thought, but then started worrying that I was wrong! I will do as you advise, in my zone I risk not getting the popcorn, but this will be better than ruining the sweetcorn, the popcorn is a whim, not a really want to have, but wanted to try.