Also what should be considerrd..., take the passage "proceeds from both as from one principle" in the context of Trinitarian theology. In order to understand what the passage means, one must necessarily go beyond it's Pneumatological significance. You're thinking only in terms of the Holy Spirit's relationship to the Father and Son. It seems you are forgetting to take into account the relationship of the Son to the Father. You can assume all you want from the ambiguity of the text in question that there are two Sources of the Spirit, but there is no way you can inject that ambiguity on the Catholic teaching that the Father is the Source of the Son. My point is that even if the Son can be mistakenly interpreted to be a source of Spirit, the Father must still be the actual Source of the Spirit, because the Father is the Source of the Son.
Read the official clarification on Filioque promulgated by HH JP2 of thrice-blessed memory (http://www.ewtn.com/library/CURIA/PCCUFILQ.HTM
) back in 1995. I very strongly recommend you read the whole thing, though I will give you a pertinent quote here:
"On the basis of Jn 15:26, this Symbol confesses the Spirit “to ek tou PatroV ekporeuomenon” (“who takes his origin from the Father”). The Father alone is the principle without principle (arch anarcoV) of the two other persons of the Trinity, the sole source (phgh) of the Son and of the Holy Spirit. The Holy Spirit therefore takes his origin from the Father alone (ek monou tou PatroV) in a principal, proper and immediate manner."
Read the original conciliar documents. The Council of Florence made a specific distinction between the term "Source" and the term "cause." In fact, the term "principle" is grammatically connected with the term "cause," not the term "Source." The Son shares as the causating principle of the Spirit, but the Son does not share in the Father's character as Source. There is only ONE Source in the Trinity according to the Latin Catholic Church, as with ALL the Catholic Churches. This important distinction between "cause" and "Source" is intimately related to the distinction between procedit and ekporeusai, on the one hand, as well as the distinction between proving the divinity of the Spirit through consubstantiality and proving His divinity through origin.
To be concise, the following two sets of words basically define the distinction between the Eastern and Western understanding on the matter, both of which are completely Catholic and Orthodox:
CAUSE, PROCEDIT, CONSUBSTANTIALITY, OUSIA
SOURCE, EKPOREUSAI, ORIGIN, HYPOSTASIS