I started running through all the combos in my head and realised it is FAR too much to post. Literrally every demigod can combo with another in a way that makes them brutal. The formula is simple. your teamate shoudl either compensate for something you dont have or compound something you do have.

Examples:

Regulus with ANYONE who can stun. He lacks one, and it will net him MANY MANY kills. Erebus is particularly good as he can both stun and chase/damage/keep sight on enemy with bat swarm. Easy picking for your snipe.

Sedna with any melee focused DG. Pretty self exlanatory. Teammate runs in, takes damage, you heal.

Same for QoT but the heal is premptive as oposed to reactive. You shield, teamate runs in takes damage on shield and you refresh it.

2 Tower Rooks. Horrible for map control but you can very easilly push into the enemies base and take the portals with that many towers. If both rooks are doing a good job of keeping towers up we are talking 10+ towers. No one in their right mind runs into that. Especially since there is a chance of being stunned.

Just a few examples. I believe the reason there have been no posts in here is because you can literally combo any DG's together. Id love to hear from others what their FAVORITE combo is. Its too much for one person to post but if we each posted our favorite, we could do a pretty good job of showing the newbies how they should play their characters with the rest of us who know how to play.

i'm a big fan of Oak and Beast.

a well timed Shield on Beast will let him get the kill and get out alive just about every time. additionally, Beast benefits immensely (even more than other DGs) from the move+attack speed buff of Surge of Faith. Beasts naturally high burst damage output is augmented well with the Penitence debuff. Beast's naturally poor health and mana regen are easily overcome by the bonus health and mana generated from Oak's Divine Justice. 

the two of them together make an amazing team that easily kills other Demigods in seconds. Oak generally takes the job of controlling the field and fights off creep waves while supporting Beast who generally takes he job of murdering enemy DGs. adding in some Priest and Sieger minion idols makes pushing through towers very easy, particularly when combined with Beast's acid spit which is also very effective against buildings. your staying power is really only limited by Oak's mana supply. if Oak has a Heart of Life you can stay out forever and push all the way to the Citadel pretty easily. 

i think the regulus + regulus combo works VERY well together
i have yet to figure out how to counter the double snipe combo when you play as a squishy character (such as UB or TB)
they can split up to cover the map but still kill so easil halfway across the map

Tower Rook and Torchbearer:  Only time a tower spamming rook has real problems is if he gets ganged up on, and who is better at hitting a whole bunch of things at once than TB?  Additionally he can stack a nice stun on them so the Rook and his Towers can pick one of them off while they can't move.  I've personally held with this combo against an entire team of 5.

gotta agree with JSZephyr on that, and also add that Erebus can be substituted with Torch in this strategy and work similarly. 

Frost Nova is a spectacular ability for keeping people stuck in Tower range for extended periods of time. Combine that with the slowing aura that an ice Torchbearer has and you can really screw people up. 

I just hate fighting 2+ regs. Not that it's impossible or unbalanced, but it makes the game VERY unfun with snipes every few seconds.