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• If the code of the node that starts as part of the 2nd roslaunch is your custom code, then reference to the ROS Parameter from the code, assuming the Parameter name can be statically defined, say folderpath.
• If the code is not your custom (I see in the OP that the node that starts in the 2nd launch is rosbag), then I'd say you'd have to add one new step before roslaunch executes e.g.

:

roslaunch 2nd.launch folderpath:=`rosparam get folderpath`

Either way, "1st laucnh" needs to start and sets the Parameter before 2nd one to start. Since roslaunch doesn't guarantee the order of nodes to start, you'd want to write a custom code to have a control over the timing to make your application robust.