The difference between this and Duel Masters is that Duel Masters players are always competing against each other, while Double Game players essentially each have their own individual quests that are fundamentally unrelated to each other. They just happen to be travelling to the same area for their respective quests, and in the process may encounter each other, whereupon they can team up and help each other, fight each other, or simply go about their own ways without interfering with each other (although if you take that last option then there's little point in playing 2-player). So yes, the books can be played as solo adventures as well if you can't find a partner.
Mainly, the unique mechanism for this gamebook mostly kicks in when the 2 PCs decide to join up and travel together, whereupon their encounters will be influenced by the individual choices of each player. As mentioned, the books can be played as solo adventures as well, so in sections where the outcomes will be different depending on whether the PCs are together or alone, the following table will be displayed:
Anyway, occasionally (but rarely) one player may have more options open to him than the other, in which case the table would be a 2x3 one instead of a square.
The Player Characters:
As mentioned, each PC has his own quest and they are basically unrelated to each other. The PC of Book 1 ("Darian, Master Magician") is an apprentice wizard trying to find the secret to transmute lead into gold. The PC of Book 2 ("Issel, Warrior King") is a warrior who was descended from kings, but his home kingdom is currently held by a tyrant whom he's trying to overthrow in order to reclaim his heritage.
The backstory that brings them together is the "Glade of Dreams", a magical glade in which adventurers can enter the world of dreams and make their dreams come true (but as you would expect, the success rate is very low). Both Darian and Issel seek to fulfil their own dreams by travelling to the Glade.
PC stats
Both characters start with 50 Skill points and 5 stats to distribute them between, each between 2-12. The thing that differentiates the 2 of them is that Darian (being a wizard) must spend double the usual amount of skill points in order to raise his Swordplay (so if he wishes to raise 4 points of Swordplay he must spend 8 points), while Issel must do that to raise his Magic.
The 5 Stats are Strength (basically HP, also used to cast spells), Agility, Luck, Magic and Swordplay. Since the stats are all between 2-12, you can probably guess that they will usually be tested via the usual 2D6 mechanism (roll lower than skill to pass a roll).
For Magic, there aren't a lot of choices - only 4 spells to choose from, an each spell has a required minimum Magic level to cast, so if the PC's Magic stat is low he may only be able to cast 1 type of spell. I will detail the spell list if there's enough players interested to play. Each spell will also cost a certain amount of Strength when used.
Combat
Combat is resolved in turn-based manner, usually with the player getting to strike first unless he's specifically said to be surprised in the text. Successful strike is determined by successful testing of Swordplay/Weapon skill rolls, and each successful hit does 2 points of damage to Strength. Enemies' Weapon skill and Strength will be displayed when combat happens. Rolling a double-2 during combat will lead to an autokill for the character that rolls it. If the 2 players are together in a fight, the one with the higher Agility hits first. If the player is fighting multiple opponents, he must split his Swordplay skill by the number of enemies for the fight!
So, before I proceed further.....are there interested players? Are there enough to make a 2-player playthrough, or should we stick to solo?