I ran into that problem a long time ago, and at that time it wasn't possible without any custom or 3rd party modules. I ended up making a quick cron that ran once a minute instead.
Put this in your crontab
* * * * * cat /path/to/htpasswd1 /path/to/htpasswd2 > /path/to/htpasswd3
Now htpasswd3 will contain both htpasswd1 and 2.
So now your htpasswd will contain this instead:
AuthUserFile /path/to/htpasswd3
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