Sharing a node between at least two layers

You need to give us a bit more information on what you are actually doing. For example: how are you storing the data and so on.

So I use simply the uMap web interface and create for example three layers. Lets call the A,B and C. These three layers correspond to three farmers, bakeries, or anything similar. Then, in the web interface I add a node corresponding to a shop (lets call the note "1"). Node 1 sells products from farmer A but also products from B and C. Later, when the map is complete, user should be able to hide the layers when they only want to see the stores selling products from farmer B. But unfortunately I add the node "1" to the layer A, when this layer is then hidden, users don't see this node anymore although it also sells products from farmer B. So the question now is, if it itpossible to store the node in all layers, so that any of the three can be hidden but the node is still visible.