Can I write "equal author contribution" in acknowledgment?
To add to the previous answer, I would consider that listing equal contribution in the acknowledgement section generally does not carry the weight as listing it as part of the front page. However, if it is clear that journal policy does not allow any mentioning of equal contributions on the title page, the acknowledgements may be your best options.
However, I think that you should have a clear discussion about this right away between the senior authors, the co-authors, and the editor. Co-authorship can have significant impact on the career of individuals, so you want to be certain that everybody is clear in what is going to happen and is in agreement with the path chosen.