If you transfer by index, you can’t interpolate. That’s what the other modes are for. So you have to use some function to map the 20 destination indices to the 10 source indices. For example divide by 2.
What takes some getting used to is that the ‘index’ node generates idices up to 20, because it’s attached to the geometry of the 20 point line. And you use that to generate the input of the index socket of the transfer attribute node, which expects indices in the range of the object you get the data from.