You cannot run with the hare and hunt with the hounds

Figuratively, ‘running with hare and hunting with the hounds’ is the deceitful behaviour of one who fights of one who fights on one side and gives secret help to the other side. Terms for these traitors to the cause are ‘double-dealers’ and ‘double-crossers’. A very old saying, dating back to the fifteenth century, is: ‘Thou hast a crooked tongue, holding with the hound and running with the hare.’ Invariably these tricksters are found out; they should be loyal to one side or the other.

See also:

• You cannot burn the candle at both ends
• A door must be either shut or open
• No man can serve two masters
• You cannot have it both ways
• You cannot have your cake and eat it
• You cannot sell the cow and drink the milk
• You cannot serve God and Mammon