Since QGIS has the possibility to make different frames, cartography has only been getting better! I often have to make map with different frames (usually because there are different scales), and I found that QGIS has really nice tricks that makes mapping much easier. One of my recent maps for instance was as below:
I always use a different font for the legend and the scale.. Although it is not possible to change the default font in QGIS, it is still possible to change it only once per feature.
So to keep the same font for the legend when you have two frames, simply create the first one and make all the customisations you want. Copy it (ctlr+C or Cmd+C). Then, select the other frame and paste your legend (ctlr+V or Cmd+V)
Tada! Your new legend looks perfect already!