Typical home wiring is 14/2. That is 14guage wire with 2 conductors and a ground.
The fuse panel is outfitted with a 15amp breaker. So when more than 15amp is drawn the breaker pops.
Theoretically if you pull more than 15amp through 14/2 wire it will get too hot and create a fire... in practical terms the wire can probably carry twice that.
If you think about power as water going through a pipe. The size of the pipe (the gauge of the wire) can only handle so much water and pressure before it bursts.
To protect a pipe you put on a pressure valve that releases the pressure when it gets too much. With wire you have a breaker.
In Canada you cannot legally have more than 4 boxes connected to one circuit, that would be 4 light fixtures or 4 electrical outlets or a combination of the two.
The concern is that as you increase power draw you get issues is in places where you have mechanical connections such as the wires that get twisted together with a meret, connected to the screw on the side of the outlet, etc. A few sparks inside a box in a wall and you could get a fire.
Things like space heaters .... 1200watts is about 11amps. If the space heater is connected to a light weight (thin) extension cord and you have a recipe for fire. That light weight extension cord is much smaller than the 14guage wiring legally designed to carry 15amps and becomes a fire risk. Two space heaters connected to the same outlet would pop a breaker. Basically you want to keep the total draw down below 15amps.
Depending on the model a slot machine will draw between 1amp and 6amps. 1amp is a 3reel unit sitting idle turned on, 3amp would be a video slot with a CRT... a LCD would be somewhere between the two. The heavy duty hopper motor will draw 3amps ..... so..... somewhere between 1 & 6.
My conjecture here is that 3 or 4 slots connected to a single wall outlet would be about the most you would want to run otherwise you risk popping breakers on a frequent basis.
If you need to plug in a series of things such as 3-4 slot machines and don't have enough outlets your best bet would be to get a power bar, that has a thick power lead,
Getting back to your question - if SLOT1 has a thick power cord, plugging SLOT2 into SLOT1 probably would not hurt other than you couldn't easily close the door. Plug in more than 3 in this manner and you will likely pop the breaker at the fuse panel just from the combined draw.