There is no need to create a separate spline for each model. Inside the Areas rollout by clicking on Plants - Select models - Pick button various models can be selected and these will be scattered based on model's probabilities (defined in Geometry rollout). With one spline for chairs and the other one for plants it's sufficient. By selecting desired ones (all chairs or all plants) in Select models window it should work fine. At first look the setup on screenshot seems to be correct.
If the issue with "randomness" persist please send us a simplified scene (containing problematic objects) to firstname.lastname@example.org. For large files, please use a web sharing service as WeTransfer or Dropbox.