As already mentioned in another post, we handle multiplayer games on octgn with the following rules:
1) Team initiative: Teammates are considered as one player. If the team has the initiative, they may move any creature in the team, it does not matter who is the owner of the creature.
(therefore it has proven very helpful to communicate with the teammate via skype etc.)
2) Shared Life: Both Mages share their life. Reason behind is that the game is not finished with a lucky round and the first dead mage. The team only loses together. The game lasts longer and therefor more exciting. That does _not_ mean, that the game lasts for 5 hours. All our games were finished in 2 hours max.
Shared life is based on the following values:
22.5 = Druid
24 = Forcemaster, Necromancer, Priestess, Wizard
25.5 = Johktari Beastmaster, Priest
27 = Beastmaster, Warlord
28.5 = Warlock
Add the two lifes together and round up.
3) Players of a Team may not cast 2 Quickcasts at a time without the other Team has the possibility to cast inbetween. Reason behind is to prevent chaincasting certain combinations, that are very frustrating.
4) No "offensive" use of Teleport on enemy Mages. Reason behind is that with the ability to teleport enemy mages, the best strategy to win quickly is always to separate the enemy team. Without the ability to teleport enemy mages, the game is more strategic and creature focused (and much more fun).
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We never had problems with building spellbooks or using certain cards, that are legendary, epic or unique. In my opinion, a team should be considered as one mage, so the team cannot spawn several unique cards (wizard tower), but they should be able to use 2 epic cards in the team (one for each mage), because they are 2 mages, not one. And of course only 1 legendary creature on the board at the same time.
Concerning the combination of the mages skills like the holy avenger pet unicorn or the other combinations mentioned above: In my opinion, they should not be used in the game. Although the abilities of making a holy avenger or pet dont say anything about this, it does not feel right for me to create these super strong creatures. Thats not the way the mages should be played.