Read this
Then this
And see how you are arguing with yourself about COMESA.
Then as I said, Kenyan companies invest in Tanzania to capture the Tanzanian market, not to gain an export proxy. Exports to SADC members, most of which are COMESA members come directly from Kenya or the Kenyan companies invest in the respective countries themselves in order to avoid import tarrifs and lengthy Bureaucracies and logistics management across borders. Trade within SADC is not tarrif free, neither is there free seamless movement of commodities just as is with COMESA so no incentives gained by investing in Tanzania for export as opposed to just exporting direct from Kenya