I suspect that the Eighth House is the House of soulmates and karma, because it's the House of Pluto, Scorpio, psychology, reincarnation, and evolution... Seems a no-brainer. Also, every soulmate I have, one of us has a Natal Planet in the other's Eighth House.
Then, take into consideration... If the Seventh is the House of marriage, love, allegiance, etc... The Second is that of values, possession, jealousy, envy, etc... The Fifth is that of sex and pleasure... Why is it that the Eighth is also all of these? Clearly, the Eighth is speaking of karmic relationships carried over from passed lives, which are often quite alluring in such ways...
So, then comes that different Planets in the Eighth feel different, and likely indicate different types of soulmates.

Sun: It might be that the two of you were once one. It is said that souls sometimes split, or divide. Because of this, it might feel very natural to join your assets together (who better to trust with your money than yourself?), and you may find the two of you greatly help one-another grow, and overcome karmic issues (who better to know what your issues are and how to overcome them but yourself?).

Moon: Likely indicates you were once family or shared a home. There's higher odds that you were mother and child, or grandmother and grandchild, or siblings (the older probably female), or wife and husband. One of you was likely a woman, a woman who deeply cared for the other. That could have been either platonic or romantic, as a loving wife could make just as much since here. But, in case of a wife it'd more likely indicate that the couple were from the same background and met at a young age, so it may have been a marriage within the family. Otherwise, it was probably family-arranged between two close families of a shared background. Also, the wife may have been the older in the pair.

Mercury: Would probably indicate that you were once siblings, neighbors, friends, or a friendlier type of mentor and apprentice pairing. It, too, could probably indicate people with whom you had correspondence (such as a pen pal who meant a lot to you), or shared your thoughts (such as academic-type colleagues).

Venus: Likely indicates someone you loved dearly, were loved dearly by, or with whom there was mutual such love. It likely indicates you were once either best friends or a loving, romantic couple.

Mars: Probably indicates you were once lovers, otherwise rivals. Someone with whom you had a very passionate relationship of some kind. In case of romance, it points to sex. In case of non-romance, it may speak of hate, fighting, and/or rivalry.

Jupiter: May indicate one of you was a man who had a great deal of power over the other. A religious figure, a father-figure, a husband-figure, etc. In case of a husband, he was a proper 'man of the house,' 'king of the castle' type. Since Jupiter is the Planet of dogmatic, institutionalized religion, he may have been an important figure to the other religiously. It's also a Planet of travel and foriegn relations... Unlike the Moon, there's pretty good odds this does not indicate an incestuous couple, as it could point to a pair who came together from two different worlds, or from over a great distance.

Saturn: Probably means one of you was a man with power over the other... A boss, a superior, a father, grandfather, older brother... It could also indicate one of you murdered the other, as Saturn is the Planet of death/endings (true death, unlike Pluto which is reincarnation).

Uranus: One of you was probably a teacher to the other, or caused a serious change in them. Or, it could be that one abandoned the other in some way, or that your soulmate meetings, partings, and relations are very sporadic.

Neptune: Perhaps someone with whom there were secrets, one or more very big secrets... Whether between the two of you in sense of both of you knew, or in way only one knew to detriment of the other. i.e. Secret lovers, secret enemies, someone one maliciously gossiped about, someone one cheated on, etc. Since it's a Planet of confusion, blindness, and mistakes, it could indicate something accidental, or something confusing or mysterious.

Pluto: One of you may have killed the other. Or, one of you may have owned the other, such as in case of a slave and their master, or a pet and their owner. Or, you may have died together, such as in the same car crash or sinking ship.

Obviously, I'm far from an expert so if one wants to explore this line of thinking they'll have to come up with their own interpretations for most possible soulmate synastries.
Though, I'd imagine we'd all agree that Venus and Mars both in the Eighth would be the most straightforward marker that two people were romantically involved, with both the emotional and physical sides of romance. Not just by the straightforward meanings of Venus, Mars, and the Eighth House... But, also considering the bond the two naturally form in the current life, which they might themselves label as a soulmate bond...