No agreement is required to send to the IRS. The agreement is what was determined when you set up the entity and as you stated, it was 50/50.
You defined your "interest in the business" when you stated it was 50/50 regardless of the fact that it may not be memorialized in writing; which it should have been.
No question participation can change, but this can't be decided after the fact based on how you want to allocate the income. The ability to file as a QJV is an option to simplify the filing process granted by Congress. You decided to go down this path. As a result, if you want to change the allocation, you need to do that at the beginning of the year (or at least prospectively), not afterwards and it needs to be a direct split, no special allocation of specific deductions.
QJV is addressed in IRC Section 761(f).
Once again, you are NOT under the partnership provisions. AND special allocations in the partner provisions are way beyond this forum and should only be done with competent tax professionals as there are complex provisions that need to be met.
*A reminder that posts in a forum such as this do not constitute tax advice.
Also keep in mind the date of replies, as tax law changes.
