I added my grass using Geometry Nodes, and now I want to add my flowers as well. I'm sure it's possible, but I'm not sure what I'm missing. When I select the flower to place on my "grass" it does not show up.
You have several possibilities to achieve this.
In all cases, however, it is important that you also give the node
Distribute Points on Faces a mesh on whose faces the points are to be distributed.
You can use the same mesh as often as you want:
You can also separate the points created on the faces into two groups and then instantiate on them:
Or you can also use multiple objects from a collection for this, and use the Pick Instance and Instance Index option to control the distribution:
PS: For all three examples there are a lot of answers here on blender.stackexchange.com. Please use the search to find the right example for you.