They both look interesting. I don't really play double kick but I think I would like something like these for the ergonomics of it. At one point about 12 years ago I actually bought a lefty double pedal so I could move my bass drum over to the left a bit and position my toms lower. I played the "slave" pedal as the primary one but it was less responsive than a "real" pedal is and I sold it a year later.
I wonder with the advances in technology since then, if these would be worth the $$$. The Off-set looks very well made and I've heard some good things about it. Might have to save some pennies and try out the Off-set. The Sleishman looks like it would take up a lot of room and it doesn't look very adjustable, positioning wise.