There are lenders who will package your loans for one combined home loan package fee so additional home loan fees are avoided for each separate loan account held with that lender.
Additional discounts may even be available dependant on the total overall home loan amount and the lender in question.
A downside to having all your home loans with the one lender could be a lack of flexibility if you ever need to refinance or access accumulated equity in your existing investment properties for further investment purposes.
As a property investor myself I prefer to keep home loans with different lenders even at the cost of reduced home loan interest rate discounting to avoid being caught with an inflexible or unresponsive lender.
There is nothing worse as a property investor to see a bargain property available for sale and to miss out on purchasing it due to a lenders apathy.