Insurance for two houses on a single lot - both rented

[Eric Andersen]

Hi there, 

I purchased this investment property a year and half ago and had it as my primary residence for a while. I had one insurance policy for both the front house and back house. The back house was listed as a separate structure (not rented). I installed a mailbox for the backhouse while there and registered a new address of 1860B at the post office. This way both parties could receive mail.

Now they are both rented (to separate tenants) and it's been an interesting experience trying to get liability coverage on the second structure. I'm being asked if they have separate addresses, have separate meters, have separate tenants etc. The lot is zoned multi family and the second structure is legal, but when I bought it and on the linked listing above, it is just listed as a single address (not A and B). The agent at Farmers is saying I need two separate policies and it's going to cost me $800 more a year than it would for a single policy.

What do you think?
Do I need two separate policies?
Did I mess up by adding a second mailbox? If so, is there an opportunity to remove the second mailbox and just have it as one property with one policy?