Yes, there's two params, H_RSC_RAMP_TIME and H_RSC_RUNUP_TIME that should be set to sort of match. I think if the ramp time is set in your ESC, then only worry about the runup time. Alitude controlled flight modes will not engage in armed state unless the runup time is expired.
I really did not see a reason, other than that, that it would not hold altitude. I guess what I would do is set the runup time to a reasonable value, take off in Stabilize (I never have liked taking off in the altitude controlled modes), increase altitude to say 6-7 meters, and engage Alt Hold. If possible center the collective stick and see what it does and try to gather some data in Alt Hold with the collective stick not moving to see if can figure out why it refuses to work properly. If possible don't move the cyclic either. Just let it drift with the wind for a few seconds to be able to see the RCOUT on your servo channels to determine if it is the flight controller commanding it to change altitude, or being caused by some other phenomenon. It could even be something like ACCEL_Z_P maybe not being set high enough for your helicopter for the collective to properly respond to a change in altitude.
It is an interesting question on the disc loading. I don't know about the efficiency of the blades in the overlap area, or how that would be calculated with a timed tandem