I have 2 bars, both with lager kegs. One is gassed with co2 and the other is with mixed 60/40. Can I use a half used keg from one bar in the other, ie mixing the gasses in the keg or do I need to de gas it?
Yes, you can do that although you may encounter some problems. The beer gas (60/40) is likely at a higher pressure, so hooking a conventionally carbonated keg may (repeat, MAY) cause it to dispense with too much pressure and cause excessive foaming. That might require reducing the serving pressure by adjusting the beer gas regulator. OTOH, it may not be a problem. I say try it and adjust accordingly if you have to.