I’m currently producing 28 backing tracks for a customer; I’ll be providing an HQ stereo mix, as well as individual stems.
My question is, can I quickly combine multiple tracks (i.e. all the strings parts) and export them as a single stem? It looks like by default Cubase is set up to export a single stem for ‘every’ track?
Run them (as well as the desired FX) through a Group Track. You can then export the Group Track. If you want to export multiple group tracks at once, just make more group tracks. Then you can export all the stems you want at once.
To add to what @rockdude9k wrote above, you might want to also check out Direct Routing and its Summing Mode which allows you to route the output of a channel to multiple destinations (such as Group Channels).
With this approach you can build some fairly complex stem setups for quicker exports.