No you can't do that on MH, no MH fares allow stopovers in domestic Australia. In fact I don't think there's any airline with a domestic leg in Oz will allow you to do a stopover domestically before you go overseas AFAIK.
Their current schedule into those 2 cities involves one 744 into Sydney and back daily as MH123/122 . A 772 into MEL and back daily as MH149/148 and another 772 that goes KUL-SYD-MEL-KUL as MH133/132. MH used to fly into MEL with a 744, as late as up to March this year. In the past they've had routings like KUL-MEL-ADL-MEL-KUL and KUL-MEL-SYD-MEL-KUL too but this was some time back.
I am not too sure if you are able to make a stopover in Melbourne when you are flying from Sydney itself. If you purchase your ticket outside Australia, you are entitled for a free stopover in Sydney if you are flying to Melboune, i.e. you can fly KUL-SYD-MEL-KUL and stopping over at both Sydney and Melbourne.
Shortfinals is right regarding MAS flights into Sydney and Melbourne. The reason they dropped the B744 service into Melbourne was because MAS wanted to fly direct into Adelaide instead of having a one-stop service into Adelaide. Hence, MAS now fly B777 daily into Melbourne, operating it as a turnaround service (as MH 148/149) and another that goes in a loop, i.e. KUL-SYD-MEL-KUL. I think they might introduce the B744 into the Melbourne route again soon since this route is doing pretty well, even when it was opertated with a B744 last time.