If a staff with a Shifting rune is transformed into a gauntlet, can you cast spells from the staff/gauntlet while wielding another weapon?

Question

Magic staves can have weapon runes applied to them. One rune is a Shifting rune, which lets you transform the weapon into a different weapon. If I use a Shifting rune to transform a staff into a Gauntlet, then pick up a sword with that hand, can I still cast spells from the staff/gauntlet in this state?

Answer 1

Probably not, and it's impossible in most cases

In general, staves cannot be etched with property runes like shifting, as explained here.

Staves are also staff weapons. They can be etched with fundamental runes but not property runes. This doesn't alter any of their spellcasting abilities.

Codas in particular are not even weapons, but instrments that can't have runes at all (so going forward I'll be talking about staves that aren't coda).

Even so, your question has merit. For example, abilities like the Champion's divine ally can add the effects of a shifting rune, and the Spellstriker Staff even has the rune by default:

Usage held in 1 hand; Bulk 1
A spellstriker staff is wrought iron with gleaming arcane sigils etched into its surface and a sharp point at the bottom. Used as a weapon, the staff is a +1 striking shifting staff.

Activate Cast a Spell; Effect You expend a number of charges from the staff to cast a spell from its list.

The activation of staves uses the charges from the staff, but this isn't a problem because even if the form of the weapon that isn't a staff, the Spellstriker Staff is still a staff in all the ways that matter for this activation: it has the staff trait, it belongs to the staves item category, and so all the rules for staves apply to it.

However, the staves' usage is "held in 1 hand", and doing so is necessary to use the activation:

Casting a Spell from a staff requires holding the staff (typically in one hand) and Activating the staff by Casting the Spell, which takes the spell’s normal number of actions.

This is a problem for a free-hand weapon like a gauntlet:

This weapon doesn't take up your hand, usually because it is built into your armor. A free-hand weapon can't be Disarmed. You can use the hand covered by your free-hand weapon to wield other items, perform manipulate actions, and so on. You can't attack with a free-hand weapon if you're wielding anything in that hand or otherwise using that hand. When you're not wielding anything and not otherwise using the hand, you can use abilities that require you to have a hand free as well as those that require you to be wielding a weapon in that hand. Each of your hands can have only one free-hand weapon on it.

Although not explicitly stated, nearly every sentence here strongly implies that free-hand weapons are not held. Especially so when you're using your hand for something else.

Moreover, the shifting rune itself could also be a problem:

With a moment of manipulation, you can shift this weapon into a different weapon with a similar form.
Activate [A] Interact; Effect The weapon takes the shape of another melee weapon that requires the same number of hands to wield. The weapon’s runes and any precious material it’s made of apply to the weapon’s new shape. Any property runes that can’t apply to the new form are suppressed until the item takes a shape to which they can apply.

In every table I've played, the notion that the new form must be similar has been understood as a reference to the activation's handedness limitations, in which case the gauntlet is a viable form. However, that notion could also be taken on its own, in which case a weapon such as a gauntlet surely isn't similar to a staff.

Answer 2

You might not be able to turn a staff into a gauntlet.

The Shifting rune includes the description

With a moment of manipulation, you can shift this weapon into a different weapon with a similar form.

While the other text is simply "another melee weapon that requires the same number of hands", some GM's may consider a gauntlet too removed from other melee weapons to be a valid Shifting choice.

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You also probably can't use the "staff" gauntlet while the hand is occupied.

From Staves

Casting a Spell from a staff requires holding the staff (typically in one hand) and Activating the staff by Casting the Spell

Of course, this is small enough that many GM's will allow this interesting use of the weapon ability. There doesn't seem to be a rule specifically about this interaction.