If you are writing into two indices, like index_1 and index_2, any document added to index_1 will also be added to index_2, so you will have new writes in the index_1 that will not be present in the current snapshot.
Same thing with updates, unless you update just one of the indices, but again, this will make your data inconsistent between the indices.
As already explained in your other post, I don't think you can achieve what you want without any downtime