Hi E.! My daughter and my niece just turned one in november and they are only three days apart so it is like twins....heheheh. Anyways, the best gift that my daughter got and that I thought was so useful was a radio flyer wagon. Her and her cousin have so much fun in it. It comes with a canopy top and seats that fold up and down. The girls each get their own seat that buckles them in and they face eachother. I am telling you they have a blast and laugh and giggle and talk to eachother. It is a bit chilly now, but this winter has been so mild and spring is right around the corner so it will come in handy. It is only one gift but it is affordable and it could be for both your neices. Honestly we forget how handy wagons are until you have one.
We have a place around us in Marlton, NJ called the Jelly Bean Jungle that is only for children 0-5 years. It was developed by school teachers. www.jellybean-jungle.com Both my daughter and neice received gift certificates so they can go together. I am not sure where you live but that was a great gift too. Maybe you can go to the website and find locations.
Good luck!
M. L.