Can I use separate handles with the X3? Or use X3 for one arm pulls?
The strongest people in the world use barbells (two hand grip) not dumbbells (one hand grip).
There is a neurological reason for this that the majority of fitness enthusiasts are completely unaware of: If your body were to interface with a heavy object in a natural environment, would you just use one hand to pick it up? Of course not, that would be an unnatural way to pick up a very heavy object. How about two heavy objects? Would you try to handle them both at the same time? Again, no. This would greatly increase the chances of injury.
To protect you from this, your body engages neural inhibition when you are put in either of these awkward positions so that MUSCLE GETS TURNED OFF and the body is limited to lifting a much lighter weight. So is lighter weight good for building strength? No. Forget independent handles.
Crossover movements are a slightly different story, since you need to go cross body for pectoral squeezing, so it’s a compromise, but independent hand movement is nonsensical in any other regard.
For a more in depth explanation:
Here are two studies that go into more details:
Researchers in 2011 observed that subjects could lift close to 20% more weight with the barbell bench press as opposed to the dumbbell press, with further research in 2012 demonstrated a 10% greater force production capacity in standing overhead pressing. This was echoed in 2012 with yet another group of researchers.
Stock, M. S., Beck, T. W., DeFreitas, J. M., & Dillon, M. A. (2011). Test–retest reliability of barbell velocity during the free-weight bench-press exercise. The Journal of Strength & Conditioning Research, 25(1), 171-177. Saeterbakken, A. H., & Fimland, M. S. (2012). Muscle activity of the core during bilateral, unilateral, seated and standing resistance exercise. European journal of applied physiology, 112(5), 1671-1678.