Well, Hard to explain it. 既是 usually used together with 又是/也是,. which means not only is/are, but also is/are 他既是老师，又是商人。 means he is not only a teacher, but also is a businessman. 既然 usually used together with 那就， which means now that it already happened, then xxx, 既然我刚才买了面包，（那）我就要跟你们谈谈法国早餐 which means now that I bought bread already, then I ..... Have you got it?

These two words are totally different. 既是 in a degree is not a complete word. It is used when you say a sentence like this: 这间屋子【既是】会客厅，【又是】卧室。（you know in China, houses are expensive, in some families, where is a small one, the bedroom serves also as a sitting room ） ------------->it's a 【既......又...】structure, means "as well as...". -------------- "是"is not always necessary. ---------------Eg. 这个小技巧既实用又容易操作。（this tip is practical as well as easy to conduct.） 既然 = since its usage is equal to "since" in "since sb has done something, ..."

既是is usually followed by words ，they are coordinative relations 。既然is usually followed by sentences which are concessive clause 。

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