as well as 变单复数是以第一个主语为主还是第二个

第一个主语为主,可以把as well as 看成是修饰的部分.作状语来修饰主语,不属于句子的主干,省去后,句子仍是完整的.
作为习语用作介词时,as well as的涵义是“还有”、“不但…而且…”.值得注意的是,在A as well as B的结构里,语意的重点在 A,不在 B.因此,“He can speak Spanish as well as English．”的译文应该是：“他不但会说英语,而且会讲西班牙语”,决不能译作：“他不但会说西班牙语,而且会讲英语”.如果这样翻译,就是本末倒置了.as well as和 not only… but also…同义,但前者的语意重点和后者的语意重点恰好颠倒.
如果 as well as用作连词引出比较从句,其义为“和…一样好”.因此,“He speaks Spanish as well as English．”应译作：“他说西班牙语像说英语一样好”.语意的重点依旧在前部,不在后半部.
请注意 as well as以下的用法：
She called on you as well as I．
不但我来看你,她也拜访了你.
She called on you as well as me．
她不但拜访了我,也拜访了你.
在下列句中,as well as表示“像…一样”的涵义：
She as well as you is an English teacher．
她像你一样也是英文教师.
这里的谓语动词用is,不用are,因为句中的主语是she,不是she和you.

参考资料：http://school.ecp.com.cn/school29/zxjs_0307/ca6967.htm