I see that the merge node only has one input but it allows multiple geometries to be connected to the same input.
Ideally I'd like to know do it using both in the python and c++ interface.

How do I create an input like that?

in a python SOP digital assets (or any other digital asset), simply set your max inputs to 11 or higher (that is the highest number of inputs that a houdini node can have). after that, Houdini switches the input plug to the multi input, and you get the white box at the bottom listing all your inputs and their order

Note that if your digital asset is a node network, you can fetch all inputs with a foreach SOP (but in python, you should be able to fetch them programmatically)

In C++ I am not sure…

grayOlorin
In C++ I am not sure…

It works the same way in C++; just set the max inputs to some high number

Thanks for the answer. In my case I want one out of 3 inputs to be a multi-input. That might not be a supported situation though since it seems like it's not a multi input but just many inputs being visualized as one.

Thanks for the answers, I'll see if I can get something to work.