As a concrete example, let's say I have a multiclassed Cleric and Wizard (Theurgy), meaning that a common spell they could both have is Cure Wounds, but let's say that I didn't have the wizard learn the Cure Wounds spell. Could the character have the "prepared wizard spells" include Cure Wounds, which is a spell that as a Theurgy Wizard he could know, but doesn't?
You determine what spells you know and can prepare for each class individually, as if you were a single-classed member of that class. If you are a ranger 4/wizard 3, for example, you know three 1st-level ranger spells based on your levels in the ranger class. As 3rd-level wizard, you know three wizard cantrips, and your spellbook contains ten wizard spells, two of which (the two you gained when you reached 3rd level as a wizard) can be 2nd-level spells. If your Intelligence is 16, you can prepare six wizard spells from your spellbook.
Each spell you know and prepare is associated with one of your classes, and you use the spellcasting ability of that class when you cast the spell. Similarly, a spellcasting focus, such as a holy symbol, can be used only for the spells from the class associated with that focus.
In the example above if it is not in your spellbook you can't prepare it as a wizard spell. See the ability arcane initiate. It is designed specifically for the purpose of adding cleric spells to your spellbook for this reason.
Short Answer: As described, you can’t
In 5th edition there are three ways that a spellcaster class determines what spells they are able to cast during an adventuring day:
- Known Spells (Arcane Tricksters, Bards, Eldritch Knights, Rangers, Sorcerers & Warlocks)
- Prepare Spells from Class Spell List (Clerics, Druids & Paladins)
- Prepare Spells from Spellbook (Wizards)
Multi-classing makes it slightly more complicated as the spells each class in the multiclass has available are determined separately.
The procedure you go through is thus:
- Build the spells available to the character for each class in the multiclass separately.
- You do this by taking the levels in the individual class and comparing them to the vanilla class table. So in the case of a Cleric 5/Wizard 8:
- Cleric 5: prepare a number of spells equal to your wisdom modifier + cleric level = wisdom modifier + 5. These spells can be any combination of the 1st, 2nd & 3rd level cleric spells available to your character. This will generally be detemined by the cleric class spell list (Pg 207 of the PHB), but some cleric subclasses also give the cleric access to additional spells not on the list (and those spells are considered cleric spells)
- Wizard 8: your character can have as many spells as they have discovered in their spellbook (including spells at a higher level than they can cast). All of the spells in the spellbook must be wizard spells (for the purposes of the Theurgy domain, only those Cleric spells added via the level up mechanic in the Wizard class are counted as Wizard Spells). The wizard 8 portion of the multiclass can prepare a number of spells equal to their intelligence modifier + their class level = Intelligance modifier + 8. These prepared spells can be of levels 1 - 4.
- As you have described the situation Cure Wounds is not a Wizard spell for the purposes of this preparation step. It would also not be present in the Wizards spellbook
- Determine your spell slots by calculating your “spellcaster level” (the procedure for this is given in the Multiclassing section of the Players Handbook [PHB]). For the example we are using this is done by:
- adding your Cleric level and your Wizard levels together. In our example this would be 5 + 8 = 13.
- Use this total (13) to read off your per spell level spell slots from the multiclass spellcaster table on page 165 of the players handbook.
To cast spells you then choose a spell from your list of spells your character has for the day (that we built in step 2), and combine it with a slot appropriate to the level you wish to cast it at (so you can use a 7th level spell slot to cast Cure Wounds if you had prepared it (using your Cleric levels) for the day).
The only mechanism the Wizard portion of the multiclass has for gaining access to Cure Wounds (if they don't take it when they choose the Theurgy arcane tradition) is to add it to their spellbook on a level up in the wizard class.