I have first node with 1000 points (no more attributes)
I have second node with 2000 points (no more attributes)

And I change some of these points in std::thread

And I have third node - Merge - first + second

How I can receive these points from Merge node? HAPI say, what there are 0 parts and 0 geos in Merge node.
How Merge Node saving all points and how I can get it in HAPI?

Houdini Session showing correct values in Geometry SpreadSheet. How I can show such points table in my own app?

You probably have figured out this already, but what you need to do is get the `P` attribute. So, call `HAPI_GetAttributeInfo` to get number of `P` attributes, this is the number of your points. Then call `HAPI_GetAttributeFloatData` to get position values.