Why don't you just put both on the charger every night then?
In terms of this product, it would be easier to charge it, but there is still the mental block around charging something which still has 90% battery. That seems wasteful to me
From everything I've read, lithium batteries actually do better when they're not discharged all the way—the only downside being that the device's battery gauge can lose calibration over time (hence the recommendation to do a deep discharge every 30-50 cycles or so).
This habit also will extend the useful life of the watch for me: Since I'm already in the habit of topping off daily, when the battery begins to lose capacity (down to 3 days from the initial 7, for instance), I won't even notice.