I would share simplyjoe's concerns that such an arrangement would be likely to attract Revenue attention, ie audits. In such a scenario they would be likely to attack the arrangement as an artificial tax-avoidance scheme, and seek to cancel any benefits gained by the parties to the deal.
Afaik, there is no set minimum number of hours worked requirement to make such a deal valid, but even if either party was genuinely working for days/weeks on end for the other, the risk of Revenue attack would be so strong that I would be very reluctant to recommend such an arrangement.
The OP should discuss this issue with their accountant. They will know the OP's situation much better than any of us.