I've done a little thinking and research and asked a few questions.
My feeling is that Arabian Parts (many of which originated in Hellenistic Astrology as 'Lots') can be used within a synastry analysis but with some qualifications.
A composite chart is very much an abstraction, as are such things as midpoints and Arabian Parts. So any Arabian Part calculated from a composite chart is an abstraction of an abstraction and therefore too remote to attach any real meaning too.
If we are to consider the Arabian Parts within a potential romantic relationship it is after other things have been looked at, such as planetary positions and aspects between charts as well as planets in each other's houses.
The Arabic Part that would be relevant would be the Part of Marriage. Or more accurately speaking the Parts of Marriage, as there are three versions. All stem from Greek Astrology.
Paulus Alexandrinus uses the Ascendant, Saturn and Venus,
For Men it's Asc + Venus - Saturn
for Women Asc + Saturn - Venus
The use of Saturn as well as Venus suggests a more formal and committed relationship.
Vettius Valens has two measures, for the first we have:
Day births (both sexes) Asc + Venus - Jupiter
night births (both sexes) Asc + Jupiter - Venus
According to Josehph Crane (Astrological Roots, The Hellenistic Legacy) This is marriage as a positive public role and uses the two benefics.
Valens' alternative is perhaps more connected to the negative side, with love leading to downfall of depresion:
For Men (day and night) ASc + Venus - Sun
For Women (daay and night) Asc + Mars - Moon
Note this measure uses Mars for women and Venus for men - the opposite sex (naturally) and uses Sun as the male light and Moon as the female light respectively. Note also that the Sun is in Fall in Libra, Venus' sign and The Moon is in Fall in Scorpio, Mars' sign.
So this is the Part to use when looking at issues where romantic or sexual relationships can cause unhapiness or even ruin lives.
Foe Positive outcome the Man's Part must agree with the Lot of Spirit (the lot of the Sun) and the Woman's Part must agree with the Part of Fortune (Lot of the Moon). Agreement here might be through sign relationships or good aspects between the rulers of the lots.
For analysis look to see which planet rules the Part, it's position, condition and aspects. If comparing to another person's chart, look to see if one of their planets is conjunct the Part (or in aspect to it) and judge by the planet and it's rulerships in their chart.
Remember the Parts may shed light on issues but they are not prime actors in a chart. The are abstractions, calculated by using relevant planets. The role of Venus in love is clearly shown by her use in most of these Parts.
Parts proliferated under the Arabs and many of the later additions are questionable, so keep to the key ones. Fortune, Spirit and a handful of others, of which the Parts of marriage, may be included.
(ok, just exaggerating a tad but still....)