Incurred in the same problem, but found a workaround - the trick is adding the marriages backward, since the source is always added to the first marriage.

1) Select the person with more than one marriage, and go to "Edit this person"
2) Go to "Relationship Events" and be sure only the last marriage is present, delete the others
3) Add the sources for that marriage
4) Go back to "Relationship Events", add the marriage previous to the one you already worked on, and add the sources
5) Continue like this for all marriages

Another solution, maybe faster (I just thought of it, but haven't tried):
in the Relationship Events tab, change the date of the marriage for which you have a source so that it's the first of the list, then add the source, then put back the correct date.

Hope this helps.