I'm not sure what it is you are trying to accomplish, but, you should never mix and match AC from difference source outlets. Creating a "system" with two AC plugs (amp + lamp) is a recipe for blown fuses and/or parts. Remember, in North America we make 120 splitting 240 down the middle. If you connect two different 120s, they might add up to 240 on you, and THEN you'll be sorry.
Similarly, if you are not using keyed plugs or there is an outlet wired wrong (which happens all the time) you run the risk of creating a dead short and tripping your circuit breaker. Or setting your house on fire if you are really unlucky.
So, what exactly are you trying to do? Are you trying to turn a lamp on and off with your leslie pedal, while still having the leslie in use? If so, I recommend creating a completely isolated circuit for the lamp, which has a TRIAC in series with it. Then trigger the TRIAC with the Leslie pedal. Or use a relay in place of the TRIAC. Using an SCR will be even simpler but would cost you light brightness (and the ability to use CFLs).