At first glance it's not a horrible suggestion (it would need to be athletics + riding, and only if you have riding) so you don't fuck over athletic builds who have no riding.
That being said, I could already do a pure cavalry build such as:
18/24
skills to atts: 14
PS - 6
Riding - 8
Athletics - 3
111 WPF in polearms
Could also go 15/27 instead, or 18/21 and have more IF or WM.
There's so many variations of builds you can do, even within the same class of character (polearm cavalry), that each one requires trading off strengths and weaknesses. Everything in the game has strengths and weaknesses, that's what makes the game so great. I think we should be leaving most aspects of the game alone, most things seem pretty well balanced to me. And they did over a year ago before the archer nerfs, and before you made pikes/long spears from 2 to 3 slots.
p.s. there's been a lot of nerfs in the game in the last 1.5+ years of me playing. I think one of the worst for cavalry was changing the riding requirements of horses. My bro was a level 33 lancer cav, who only had 4 riding for his courser. They moved it up to 6 riding and completely fucked him over (obviously no free respec was given). Please leave c-rpg alone...every nerf and buff has an effect on every other class in the game...and usually it's unintended and doesn't seem to be anticipated by the balance team or the devs. I think the riding requirement changes were needed, but you should have let people respec for free. But my point is that if you implemented the changes that the OP suggests, you would fuck over a lot of players who were never going to retire again.