This functionality is being considered for a future release, but for now the only reliable way to accomplish it is to have a post-process node handle it in code, or put a linkable cell node after the optimized communication node and do your split by percentage in a follow-up (non-optimized) campaign (link > split > cell/cell > comm).
If you try to force the logic into your optimization scenario, you'll either:
have potentially biased results (since the higher value customers will likely go into the larger cell, since that will result in a higher objective value)
get an undesired result (if the input size is < 60,000 total and you specify 40,000 and 20,000 respectively as maximum constraints) since there's no guarantee the split cells will have the correct ratio
have infeasible constraints (if your input size is < 60,000 and you try and specify 40,000 and 20,000 respectively as minimum constraints for two other cells)
have to frequently maintain the results (if the "main" cell size changes from 60,000)
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