并列关系就是Webster老师所言，legacy和两个what从句并列。

what不能引导定语从句，因为作为一个融合关系词，本身包含了先行词，所以此处what引导的只能算一个作表语的名词性从句(也就是所谓的表语从句)。

把句子改一下:

Heritage is our legacy from the past, which we live with today and which we pass on to future generations.

这时候，which引导定语从句。当然这里有两个定语从句，并列修饰先行词legacy:

Heritage is our legacy from the past, which we live with today.

Heritage is our legacy from the past, which we live with today, which we pass on to future generations.

不过句意都差不多。

这句话在联合国教科文组织的网站上可以看到。