Never seen the Twinfeeder, interesting concept. I just mixed both pellets and flakes into the same compartment. But I have to crunch down some of the flakes or it'll block the outlet of the feeder. If needed, consider making a feeding ring out of RODI tubing and position the outlet of the feeder over the ring. So the food doesn't rush into your overflow...
Both worked well and accept the same pellet/flake food. The LifeGuard is easier to mount to the tank and I would recommend it over the Eheim if you have a rimless tank.