Many people feel the same, including myself. Throughout the years, I have acquired the following devices which help me to hear my bass well without playing (too) loud.
1. IEM (headphones). I understand this is not the path you want to take (at the moment) so I will not elaborate on it any more.
2. Tactile transducers (Eich Bassboard, Eich Drum chair, Backbeat). These devices give you the feeling of low frequencies without anybody hearing them. They move your body and your brain tells you that you hear the lows. They are primarily made/marketed for players using IEM. I am using them both with and without headphones though. When I use one of them with a cab (without IEM) it allows me to drastically decrease the volume of my cab...plus I cut bass a lot. That way, the tactile transducer provides me with lows and the cab provides me with mids. On Saturday, I played at an outdoor festiwal. The stage was huge. I used Eich Bassboard and a tiny 1x12 cab. It felt like standing in front of a wall of Ampeg fridges.
3. Last but not least: Ear Box
I use it with a cab usually (as recommended by the manufacturer). I put the cab on the floor and I place the Ear Box on mic stand next to my ear. The cab supplies the lows and low mids while the Ear Box supplies the high mids and highs for my ears.
Sometimes I use it without a cab. When we play in small rooms with one of my bands, the stage bleed is a significant issue. So I came up with this solution: I sit on the Eich drumchair and use Ear Box to hear high mids and highs. No bass cab on the "stage" means no stage bleed from bass...
Once, I even used the Ear Box on its own: we played in a large club with huge subs...so I knew I would hear the lows from FOH.
Probably not. But it serves me well. What I consider important is to use LPF with the Ear Box-it allows me to cut the harsh highs and hear the high mids... IME, the Ear Box allows me to decrease volume of my cab a bit (not a lot).
I don't think it would work well. If you use two sources of low frequencies, those two sources can compete with each other creating various issues...
To conclude:
1. IMHO, Ear Box is a better (and cheaper) solution than second combo amp on a mic stand.
2. If you want to decrease the volume of your cab a lot while still hearing yourself well, you should consider tactile transducers (especially Eich Bassboard).
Good luck!
Links:
EICH Amplification
Eich DBC400R | EICH Amplification
BackBeat - Play Bass, Feel Bass