I have grown both peaches and plums before but not on the same tree. My thoughts were to have three upper limbs of peaches (Contender) and about three lower limbs of plums. It has been awhile since I grew peaches but as I remember it about 1/3rd of the previous year growth needs to be removed and I think if these limbs are the upper ones that they would possibly have enough vigor to grow back out each year. I’m limited on space. Any thoughts or comments are welcome.