The 40A is the rated capacity of the ELCB. It really depends on the connection paths of your house wiring. When your electrician says splitting he probably means parallel connection, under full load the split contains 31.5A+31.5A = 63A, that's why he says 40+40 is enough.
But this is under assumption that on any one of the series part, your max current load is only up to 40A. For residence under normal usage (e.g. one plug connects to one device) this is usually okay. The only danger comes when some people like to abuse their power sockets, like connecting multiple high-load devices using power extension cords and so on.