If I follow correctly, this isnt really a Project problem.

From what you say, you have 2 resources (man A and man B, say) who both spend equal time on each task. The cost for each is £A and £B. You have an amount of money- the fee- call it £X. You want to know how many hours man A and man B can put into each task without the fee being exceeded.

If this is correct, I would suggest excel as a better alternative. If man A and man B put the same amount of time into each task, then the cost for each hour of the task is £A + £B (both are working on it so the total hourly cost is found by adding them together). You then have £X that cannot be exceeded. Divide this by the total hourly cost , so you have £X/(£A+£B) and that tells you how many hours they can spend on each task.

If I have read it wrong, please clarify and we will see if we can sort it out