this move seems impossible unless its in a tag, i can never do it, been trying for hours, is it even possible ? would appreciate tips if it is, about to give up 
You’ll only be able to land Cr. Hp xJCx J.Hp in three different cases:
- As you stated, a Cross rush tag in or switch cancel if you have the proper setup.
- Anti Air situations
- Corner only off MK or HK senpusha juggles. MK being the easiest to land and it is timing sensitive of course.
Some characters fall faster than others or are just smaller in nature. Like Cammy will be harder to hit than Ryu or example, but both are do-able.
All in all just work on your timing against Ryu or Ken as they are the average build, resist the urge to mash it out – that being said when I’m trying to figure the timing out on moves I do typically mash because it helps me “see” the window I really need to hit it. So do what works best.
Rayartz pretty much hit this, but out of curiosity, what character and move are you trying to cr.HP xJCx j.HP with?
I ask because in example, this is easy to hit off of say, Cammy’s or Julia’s DPs (literally have all day to time it in some situations if the DP hits high enough) or Law’s SSK, but I have a hell of a time trying it from Julia’s launcher (since she launches so low). So that may be a part of the problem for you as well.
^this. Depending on what you’re trying to land this combo off of, you may want to do cr.hp xx JC > shikusen > fs.mp xx lk.senpusha instead for consistency… less damage but easier to do off low juggles.
Yeeeup, that’s what I tend to do when I need consistency, though I do cr.mp instead of fs.mp… actually is there a difference between the two in a juggle combo?
fs.mp makes it easier to combo a store as it moves you forward.
Not that I’ve noticed. I tend to lean towards Cr. Mp as well; call it habit from SSF4 is feels the most natural.
But Fs.Mp xx Hk fuhajin can get you some higher rebound for bringing characters back in. At least at the proper height I need for Chun anyway.
Key is to hit the 2nd hp after the jc when the character is at the peak of the juggle.