First, find a date that will work with the sellers. When I have a brokers' open, i put it into the MLS first. I also notify the office and a bulk emial is sent out to all of our agents. Using the MLS I complied an email address for all the surrounding town agents. I use this list to send all the agents notice of the open house.
I know with my office, brokers opens are held onTuesdays. This way people can travel from house to house. See if there is a common day for the office or try and partner up with another agent who has a listing close by. If you have 2 properties in a close area, you may get more agents out.
Best of luck!
Realty Executives, Pequannock