I want to make my Templar (modified Paladin) into a mounted knight using Animal Companion and Command. Could I use a single instance of a multiclass move, or would I need to use two moves?
You would normally need two multiclass moves to take two moves, but Animal Companion and Command are starting moves that don't do anything without the other, so they depend on each other. In that case, they count as one for multiclassing (p. 31):
The multiclass moves allow you to gain moves from another class. You get to choose any move of your level or lower. For the purpose of multiclassing, any starting class moves that depend on each other count as one move—the wizard's cast a spell, spellbook, and prepare spells for example.
However, those two moves cover more (and are missing other things) than what you need to emulate a Paladin's holy mount, and they're definitely overblown for just a mundane mount but with a move to represent training. The Dungeon World Way here is to write a custom move. Especially since you're already working with a modified character class, a custom move is the most appropriate way to handle this. Talk with your GM and figure out what you want it to do, and consult the Advanced Delving chapter for advice on custom moves.
Alternatively, you could use already-created custom moves for handling mounts. The 3rd-party supplement Mounted Combat adds rules for mundane trained mounts (both in combat and out of combat, despite the name). I've found the rules themselves to be a fairly lightweight addition to the game, and can be used by any kind of character to model mounts using the new moves and mount statistics. (My only caveat is that the statistics for mounts overlap a bit with the new statistics and moves for followers in The Perilous Wilds 3rd-party supplement, so if you try to use those together and have any mounts modelled as followers, you'll have to resolve that conflict. I'm still experimenting with how to best handle that.)
This is too long for the comment forms.
Here's the move I made:
If you gain a special bond with your horse, you may take this move instead of your class move, next time you level up.
You have a supernatural connection with your loyal horse. You can’t talk to it per se but it always acts as you wish it to. Name your mount and choose a base:
• Ferocity +2, Cunning +1, 1 Armor, Instinct +1
• Ferocity +2, Cunning +2, 0 Armor, Instinct +1
• Ferocity +1, Cunning +2, 1 Armor, Instinct +1
• Ferocity +3, Cunning +1, 1 Armor, Instinct +2
Choose as many strengths as its ferocity: Fast, burly, huge, calm, tireless, intimidating
Your horse is trained to fight humanoids. Choose as many additional trainings as its cunning: fight monsters, labor, travel
Choose as many weaknesses as its instinct: Flighty, slow, broken, frightening, forgetful, stubborn, lame
When mounted on your horse…
• …and you attack the same target, add its ferocity to your damage
• …and you track, add its cunning to your roll
• …and you take damage, add its armor to your armor
• …and you discern realities, add its cunning to your roll
• …and you parley, add its cunning to your roll
• …and someone interferes with you, add its instinct to their roll