The problem using SIO2IDE on the 800 is the delay while the SIO2IDE is initializing. Since you are powering it from the 800, it is not ready when the 800 tries to boot. Once the SIO2IDE is ready, you need to do a cold boot, which the 800 reset key doesn't do. If you are in BASIC you can type X=USR(58487) once the SIO2IDE is ready and it should then boot from it. Without basic you would need to perform one of the hardware mods available to make the 800 RESET key do a Cold Start.
I believe there is a way to connect the SIO2IDE's "Reset Out" line to the SIO so that the computer will wait for it to finish initialization before the computer boots, but I've not tried this myself.