In Commander, if you have two commanders in the command zone, can you cast both in the same turn?

In Commander, there is no rule preventing you from casting both of your commanders in the same turn. The Commander rules allow commanders to be cast from the command zone as a special action, and nothing restricts casting multiple commanders in a single turn as long as you can pay for them.

Each commander can be cast from the command zone by paying its mana cost plus any applicable commander tax (an additional {2} for each previous time that commander was cast from the command zone, per Commander rule 903.8). These are separate spells, each requiring their own timing window — typically your main phase when the stack is empty, following normal sorcery-speed rules (CR 307.1).

You must have sufficient mana to pay for both. If your first commander is countered and returns to the command zone, you could even cast it again (with increased commander tax) in the same turn, as long as you have the mana and timing permits (CR 903.8).

Concrete example: You're playing a Partner commander deck with Tymna the Weaver and Thrasios, Triton Hero. On your first main phase, you cast Thrasios ({1}{G}{U}), then after it resolves, you cast Tymna ({1}{W}{B}) in the same main phase. Both are now on the battlefield — this is perfectly legal.

Remember that each commander is tracked independently for commander tax purposes (CR 903.8). So if Thrasios has been cast once before, it costs {1}{G}{U} + {2} extra, while Tymna would still cost her base cost if cast for the first time.