I feel stupid asking this - SIP in old portfolio or new?

So i have 10 lakhs already invested in Mutual Funds and i want to start an SIP of 50,000 rs monthly. So my ultimate question is shall i start a new SIP altogether or invest in the old portfolio as my thinking is the old portfolio has already a great corpus and investing in it would help to grow the portfolio better as the corpus would increase month by month. Very new to MF and all this is new so please make me understand this case of mine. What would be beneficial? OR both will yield the same returns?