.debug bg = Toggle debug mode for battlegrounds. In debug mode GM can start battleground with single player.
Or if in your case if you want to do a true 1 vs 1.
You can temporarily modify world.battleground_template with min players per team to 1 for that particular battleground.
https://trinitycore.atlassian.net/wiki/display/tc/battleground_template#battleground_template-MinPlayersPerTeam
1. .reload battleground_template
2. level 80+ alliance queue up for that BG.
3. level 80+ horde queue for the same BG.