is it possible to communicate between two ROS systems, exploring in one, and SLAM in the other ?

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The short answer is "yes." Take a look at http://wiki.ros.org/ROS/Tutorials/Mul... to see how to set that up.

In practice you'll need to be careful about managing the data flowing over the network. You should run roscore and the motor controllers and sensor drivers on the RPi so that your robot is responsive and immune from network hiccups. On the desktop you can run SLAM and exploration; these consume lots of CPU but generate small messages like tf transforms. The challenge is getting the sensor data up to your desktop reliably. Laser data is pretty small, but pointclouds can be huge, so you'll probably need to downsample on the RPi. It really depends on your situation.