Yeah, those double HCBs are tough and there’s not much leniency as far as hitting the directionals - I’m using a stick but I still find the double HCBs difficult, especially trying to 2-in-1 into it, that’s very hard…
Two things that I’ve found which have helped me a little:
Trying to be conscious to make sure I’ve got the stick all the way to the last directional input before pressing the buttons. When doing HCBs, the forward, down and back directions must be hit, therefore, it helps to be mindful to wait until you’re firmly on the last directional before you hit the button(s) to execute the special move. For quarter circles, I find it helpful to make sure I’m hitting the diagonal in the quarter circle motion as the move will not execute without it… You won’t get any QCF or QCB moves to come out if you don’t hit the corresponding diagonals.
Use the button buffer!!! It really helps you get the fastest possible timing on your specials and this can be so helpful when doing some of the more difficult combos, especially those with tighter timing; unfortunately, the button buffer doesn’t help you with the timing of your normals (like when trying to link normal attacks) as the buffer for normals is only 1 frame. However, when you’re doing specials, you get something like a 13 frame buffer!!! (The buffer for supers is something like 17 frames!) For example, if you’re using quick, consecutive specials to juggle your opponent (inside or outside of the corner), after doing one special, you can perform the motion for the next special and the hold the attack button down. As long as you time it to be within 13 frames of your recovery at the end of the 1st move, the next move will come out at the earliest possible moment. This also helps a ton when drive cancelling because holding the button will ensure the Drive cancel will occur with proper timing as long as you’re within 13 frames of the cancel window. Trust me, this helps a lot.