This exercise is not for the faint of heart.
Supposedly, there are 2 small uke like instruments to play the kerong and cong part together; hence the name Keroncong. I can't find any photos or youtube videos.
I did find some instruments on sale on the internet. We had to fit a big taxi into a very very narrow Jakarta residential street and I thought to myself, "what in the world have I got myself into?"
These are not the best made instruments but they were cheap - about $45 each and they are a sample of what Indonesia Keroncong is.