For the most important parts of sacraments, there are certain elements which are similar to two religions. These are interrelated and in fact the first was the precursor of the newer, which evolved from certain historical events and necessities. The older faith is still very much stricter in its enforcement of rules while the other is more ecumenical.
Ecumenical concerns have made Christianity that much more open to the influence of other religions major or minor. It means that the use of the Jewish and Christian wedding rabbi may be relevant to both. And while the rabbi is an expert or an ordained person who actually is relevant to only Judaism, Christians these days want this to officiate on their weddings.
Thus the combinatory effect is not lost on those who want there to be a unification of all world religions. The Dalai Lama was the first to signal this call, while Christianity was merely content to say that is was open to other influences. The call by the Tibetan religious leader is bolder and more to the point.
There is no need any more for religions to quibble over points of faith much less practice. Practitioners may be strict but there is often some similarity that could bind all together in one ceremony. The rabbi is a singular person, and therefore could officiate in any given way or a number of accepted ways.
There are also older ritual in the church of Christ that are related to older traditions in Judaism. And these the rabbi will understand very well, especially when it comes to binding a couple together in faith and love. This is more of a preference for Christians although for Jews it remains to be a requirement they cannot do without.
For more items which are going to be needed, there might be things that the Jewish needs to use other than the ones used by Christians. It may be something that is more useful for ceremonies too and sometimes Judaism is entirely more practical. And while Christians are open sometimes they will lack the older and actually better principles.
Thus the combination is often something that completes or rounds out the process. Soon perhaps even the Jews will see how there is much of the priestly work that could work for them. And while they can have more complex rituals and really hard things to fulfill, this could be something that works out well.
For those needing the kind of services that are older, it is good for the Judaic precepts to stand out. There is enough of it in the Old Testament and most Christians will be familiar with these. And thus they are going to be actually receptive of what the rabbi says than most others.
The fact that Christianity grew out of the older faith of Judaism is important. And for many it means that there are some things that will work well together. And for some time there has been a movement that moves central to the unification that the Dalai Lama was talking about.