Can I plug an air conditioner and a chest freezer into the same outlet?
This is a 20amp outlet, but before you go drawing 16amps/80% of that, you should verify that there is a 20amp breaker/fuse on the other end as well as 12 gauge wiring (thicker than the standard 14 gauge you find on most 15amp circuits). If there is only a 15amp breaker, then the max you can draw from that outlet is 12amp (80% of 15amps).

You mentioned your one device pulls 8amps - that leaves at minimum, 4 amps available for your freezer - what is the current draw of that device? If its 4 or under and the breaker is 15amps then you are OK.  If it is over 4 then you have to ensure you have a 20amp breaker in which case the max you'd want to pull is about 16amps combined.

If it is a 20amp breaker, then you will need to find a power bar that is rated for 20amp - I think most of the time they are rated for 15 but if you look hard enough you will probably find one for 20.  You could also replace that one outlet with a 2 recepticle outlet which would eliminate the need for a power bar.