Say I want to cast Dialing the Lucky Number, and I have Fate 2 and Forces 2, can I form a cabal with a mage that has Space 2 and cast it with him?
If not, is there some other way to combine Arcana between mages?
From page 79 of that book:
With Ritual Synergy •••••, the mage leading the ritual need not know how to cast the entirety of the spell; other participants with this Merit can contribute their personal Arcanum knowledge to conjunctional or combined spells. All participants roll their relevant dice pools (using the Arcanum they are contributing) and spend a point of Mana.
Example: Adept Solis wishes to cast a conjunctional Fate 4 + Space 2 “Gift of Fortune” spell (see p. 157 in Mage: The Awakening). He knows Fate 4 but not Space 2. He teams with a fellow mage who does know Space 2. They each know the Ritual Synergy Merit at five dots, so each can contribute his personal Arcanum knowledge to the whole of the casting.
Otherwise, no. Each contributing mage must have the ability to cast the spell to take part in a group ritual. Ritual Synergy ••• allows teammate mages who can't cast the spell but have at least one dot in the relevant Arcanum to assist, and Ritual Synergy ••••• works as above.