Hi SelfProclaimedPrincess,
From what I have read and gathered it really depends on the maturity of the people involved. I just realized recently and ex and I both had Moon/Pluto aspects and I myself am very Plutonian. Long story short, we did not work out, and I think I in the relationship relied too much on him for my self-esteem and was emotionally demanding. My Moon/Pluto conjunction was opposite his Moon in synastry.
However, it depends on the whole synastry chart and all the aspects on top of the maturity and self-awareness of the people involved. My relationship I just mentioned above was in my teens into my early 20s so granted it is not going to be mature. We also had some Neptune synastry which does not bode well.
In this relationship you mention are there Pluto synastry aspects or are they simply two Plutonian people? Further I would look into all of the synastry and weigh all the aspects together to determine if it will more than likely work out or not. Perhaps also look into current transits with both charts to see where their heads are at in general at this point in their lives, to see if they will be able to be committed to a relationship right now.