Disagree with most of the people here TBH. I work in a PE fund and we look at synergies with portfolio companies (and with the fund itself) all the time.
The definition of synergy is that there is a positive or negative value when you calculate the valuation of an entity and when you compare it with the valuation of the sum of the parts
Example of positive synergies in this case can be reducing operating costs because the PE owns a company that produces the same input the target uses, and thus can be procured with a discount.
Example negative synergies include most strict (from the PE side) regulations that the target just adhere and doesn't at the moment. For example, when Cargill buys a company, it must adhere to the whole sustainability framework that Cargill adheres, and is costly when compared to adhering to the minimum FDA framework
Hope it helps