Only exclusive content won't merge, all your other items such as skins, heirlooms, non-exclusive packs, legends, currency and all the other stuff should merge over just fine.
If both these platform accounts are connected to the same EA account then there should be no problem.
Some of the exclusive content won't be shared between platforms because well, they're exclusive.
The companies that own these platforms have decided that they want to keep these items exclusive to their platform, EA pretty much had no say in this from what I know.
I do agree that it's a bit unfair of them to make players able to pre-purchase the season pass for the next season before letting players know that cross progression was coming out in the next season. They definitely need to learn in that aspect, as I do want to think this was a honest mistake rather than them using a cheap trick like this to get a few extra bucks.
Would be great if they would refund players that have purchased both the season passes after merging, though I doubt that'll happen.
But accounts will merge in waves, so that means when S19 rolls out, not all accounts will have cross progression enabled yet.
It might take until the middle of the season to merge all accounts (given that there are no issues), though I don't have any exact projections either, so I'm not even sure whether or not they'll be able to successfully roll it out in the entirety of this season either... I guess we'll have to wait and see.
