I have never been a big believer in Rooney and Van Persie as a partnership but I think Van Gaal can make it work. The real issue with the Rooney/Van Persie partnership is that nobody seems to have given either clear instructions about how they should work together. Both Fergie and Moyes seemed to think that they could work it out on the field but Van Gaal is too meticulous for that. Van Gaal will give them both a very clear, very definite role and demand they stick to it. That will help them not to step on each others toes, and know how they should play off and with each other.
Van Persie spent weeks with Holland acclimatising to the #9 role in Van Gaal's 3-5-2, he won't need to learn it again. Plus, throughout pre-season, Rooney has being doing the role Robben played in the Dutch World Cup team. Its early days, but to my eyes, the Robben role has brought the best out of Rooney too. Provided they stick to their defined roles, and Van Gaal won't tolerate anything else with his match cameras tracking their every move, Rooney and Van Persie will function effectively together.
PS) I think a lot Rooney's improvement this pre-season has come from having a manager who gives him unmistakeable orders. There's no talk about playing off the cuff or relying on instinct. Van Gaal has a clear way of playing, all the players say so, and he is just putting square pegs into square spaces according to his philosophy. Rooney's loving it. He's not wandering off around the place, when he drops in Mata instantly exchanges with him. Rooney doesn't have to worry about how things are meant to go and as a result the hesitancy we saw from him last year is gone.