Well for starters, you wouldn’t want to combine herbs with contradictor actions, so like you wouldn’t want to combine herbs that lower blood pressure and herbs that raise blood pressure.
When it comes to tincturing, most herbs are fine but you don’t want to mix herbs that are high in tannins with those high in alkaloids, the combination can cause a chemical reaction that looks like glue. As far as I know, you can create a tincture of the herbs together, but you can’t combine two separate tinctures or else you get the glue issue, but to be safe I would just take them separately and avoid the issue altogether