Один пирог два раза не съешь

You cannot enjoy the advantage of two incompatible activities

Cf: If you sell the cow, you sell her milk (Br.). If you sell the cow, you sell the milk too (Am.). The sun doesn't shine on both sides of the hedge at once (Am., Br.). You cannot burn the candle at two ends (Am., Br.). You cannot have it both ways (Am., Br.). You cannot sell the cow and drink (sup) the milk (Br.). You cannot sell the cow and have the milk (Am.). You can't eat the same bread twice (Am.). You can't eat your cake and have it (Br.). You can't eat your cake and have it too (Am.)