Talisman - Do war horse effects stack?
Flavorwise, the idea of riding multiple warhorses definitely falls flat.  However, there is nothing in the rules indicating that only one is allowed at a time.  The power is held somewhat in check due to a single loss of life in battle/combat resulting in losing all of the Warhorses.
While not answering for this specific card, the FAQ indicates that multiple &quot;Add Craft to Strength&quot; effects can be applied to a single battle:

Q2: Can the Monk use his inner belief to add his Craft value to his
Strength during battle and then cast the Psionic Blast Spell to add
his Craft value to his Strength again during the same battle?
A: Yes

To alleviate the flavor concerns, I like the response from this FFG Forum Post:

It seems to me that many people think it's weird because a human being
can only be mounted on one horse at any given time.  While this is
true, who says that the player is mounted during combat?  Sure it
makes the most sense, to have the horse biting and hoofing while its
rider slashes with his or her sword, but there's no reason that a well
trained warhorse couldn't be flanking and opponent with its faithful
companion.  When you consider this, two horses, or three or four or
whatever, are irrelevant.
But this is all superfluous, anyway.