Q:

If 2 creatures in range have the Sanctuary spell cast on them, can you repeatedly switch targets between them until you succeed on the save?

The description of the sanctuary spell states:

You ward a creature within range against attack. Until the spell ends, any creature who targets the warded creature with an attack or a harmful spell must first make a Wisdom saving throw. On a failed save, the creature must choose a new target or lose the attack or spell. This spell doesn't protect the warded creature from area effects, such as the explosion of a fireball.

How the spell works is quite obvious when only one creature is warded at a time.
However, I am wondering how this would work when two creatures are warded and both within range. Could a monster switch between targeting either of them until they succeed on their Wisdom saving throw?
As an example: two clerics - Alice and Jasper - are ambushed by an powerful vampire named James. They are fiercely opposed to killing, and so instead of harming James, they both cast sanctuary on themselves. James attempts to bite Alice, but fails his save. He then switches his target to Jasper, but fails his save again; now what happens?
The spell seems to state that he is to choose a new target, and does not appear to prohibit targeting Alice once again (Alice's ward is a completely separate effect to Jasper's ward). However, that does seem to defeat the purpose of the spell.
So, what happens RAW? And RAI?

A:

It depends what one means by 'new'
In order for the problem you mention to be a problem, 'new' needs to mean 'different from the current one'. If new means instead 'that you haven't picked so far' there's no problem.
I think the unproblematic interpretation of 'new' is slightly more common in ordinary usage, and thus more the "rules as written" with the very-problematic natural language convention thingy. Neither interpretation violates the RAW without that convention, however.
Regardless of the RAW, I've never seen anyone use the problematic interpretation here-- new, in this context, always means that you haven't yet tried to target this attack.

A:

'New' is defined by the attack itself

Until the spell ends, any creature who targets the warded creature with an attack or a harmful spell must first make a Wisdom saving throw. On a failed save, the creature must choose a new target or lose the attack or spell. This spell doesn’t protect the warded creature from area effects, such as the explosion of a fireball.

"must choose a new target"
A new target of what?
Of the attack or spell
If the vampire attempts to bite Alice (targets, fails save) and then attempts to bite Jasper (targets, fails save), the vampire must find a new target of THAT bite attack or else lose THAT bite attack.  Alice is not a valid "new" target, because she has already been an attempted target of THAT particular attack.
If the vampire then uses its Multiattack to make an unarmed strike on the same round on Alice, that would be permitted because it would be an instance of a new attack (and would provoke a new save).
Related:  Does a creature attacking a target of Sanctuary lose their attack/spell if there is not another target immediately in range?

