Complex sentence: Since the inception, all engineering was Wilkinson's responsibility, and all sales and business was HH Franklin's responsibility, and there was a tacit understanding that neither would interfere in the other's territory.
Split into simpler sentences: Since the inception, all engineering was Wilkinson's responsibiltiy, and all sales and business was HH Franklin's responsibility. And there was a tacit understanding that neither would interfere in eachother's territory.