How to combine content from two different environments?
Node Export
The Node Export module might be used for this. Here is an excerpt from its project page:

This module allows users to export nodes and then import it into another Drupal installation, or on the same site.
Using this module you can save yourself a lot of time setting up new websites that have similar nodes to websites you've already made, migrating nodes to new Drupal versions, or between development/staging/production sites.
You will need the same content types for the imports to work (unless using Feeds to import), and ideally relevant compatibility with fields, and modules.

Feeds
Another option that might work is to use the Feeds module. Here is an excerpt from its project page:

Import or aggregate data as nodes, users, taxonomy terms or simple database records.

Bundle copy
Yet another approach is to use the Bundle copy module. Here is an excerpt from its project page:

Has export/import support for:

Node types
Taxonomy
User
Field API fields
Field groups

You can find the export/import links for the supported entities on following paths:

Nodes: admin/structure/types
Taxonomy: admin/structure/taxonomy
Users: admin/config/people/accounts