How to define multiple name tags in a struct

What you need to do is to use space instead of commas as tag string separators.

type Page struct {
    PageId string                 `bson:"pageId" json:"pageId"`
    Meta   map[string]interface{} `bson:"meta" json:"meta"`
}

It says in the documentation of the reflect package:

By convention, tag strings are a concatenation of optionally space-separated key:”value” pairs. Each key is a non-empty string consisting of non-control characters other than space (U+0020 ‘ ‘), quote (U+0022 ‘”‘), and colon (U+003A ‘:’). Each value is quoted using U+0022 ‘”‘ characters and Go string literal syntax.