Combine language-specific settings for multiple languages in Visual Studio Code
Old question, but I was just looking into this and an answer here would have saved me some time.
Your approach is the correct one, and is the only option other than specifying word wrap globally. To answer your question directly, there is no way to combine them. In fact, this is how language-specific settings are specified in VS Code's default configuration.
As of version 1.63, this is now possible as follows:
"[plaintext][latex]": {
"editor.wordWrap": "on"
}
https://code.visualstudio.com/updates/v1_63#_multiple-language-specific-editor-settings