As far as I know it hasn't been FAQ'd anywhere.
As it is, I understand your question, though I think it may need to be written to conform more fully to the FAQ request guidelines (which I am not intimately familiar with myself).
In any event, by my reading you can apply more than 1 metamagic to the spell being cast, but can only apply 1 using the staff. Why would a spell be restricted to only 1 metamagic when using the staff, when you could regularly cast a spell with as many as you can fit on until it uses a 9th level slot. Though, this is what I would consider the rules as intended.
Rules as Written, it does specifically say you cannot apply more than 1 metamagic feat, not clearly distingusihing between using the staff or your own spell slots to do so.
RAW the wording allows you to only apply one metamagic feat to the spell "IN THIS WAY". In this way is referencing the two sentences before it, which describe how the Staff adds metamagic. You are perfectly free to add metamagic to the spell in other ways per the Staff, just not in the way previously described. For clarity here's the text:
In addition, this staff can be used to cast spells using any metamagic feats known by the wielder without increasing the spell’s level. This consumes a number of charges equal to the number of spell levels increased by the feat. No more than one feat can be applied to a spell cast by the wielder in this way.
In this way is the key bit of text there, because without then yes, you could apply only one metamagic feat to the spell being cast. However, RAW the Staff only wants you to apply one metamagic feat to the spell by spending its charges, which is useful clarifying text because otherwise based on the first and second sentence alone, you would be able to RAW add as many metamagic feats as you had charges to pay for.
RAW/RAI is pretty clear, the Staff lets you add one metamagic via charge consumption and does not restrict the spell to only one metamagic feat.
