I used to do exactly this : collect several canvases, create a new one, split it, and “drawclonepad” the initial canvases on the subpads. However, I get a memory leak with drawclonepad as reported in this other thread : Memory leak in TCanvas::DrawClonePad
Therefore, I thought I would try a different approach meanwhile we discuss the memory leak. And I found this issue with “Append”. I definitely think that it is a bug but I agree that it might not be the highest priority 
Thank you
Barth