How would a secondary be as good as ur main. SECONDary. its a back up character. They're only used for certain characters and stages. so im not gonna put the time into them like i would falco for most or all match ups and stages, just a few. Thats why they r a secondary.
You still need the control over your secondary to be nearly as good as your main. Without that, it's rather pointless to CP a character when you will just go and lose with them due to less skill with them.
If your secondary is as proficent as your main your CP match will instantly be in your favor. That's because you rule out skill as being the limiting factor in your advantage, and it just comes down to your CP, character, and how well your opponent handles the disadvantage match up.
If your secondary isn't as good as your main, you might actually be playing a match that while is supposed to be your advantage, is really a disadvantage because your skill isn't enough to overcome the other characters skill.
So while your secondary may not have to have the same effort put into as many match-ups, you certainly need to have the feel of your secondary be just as good as your main. This is espically true if your secondary can only CP a character through dittos. I see that happen quite a lot when someone has a main that loses to MK badly. Their solution to the problem is to use MK as a CP. Which if your using MK (secondary) against someone else's Main in a ditto, the mainer of that ditto should be victorous. So that is an example of why you want your secondary to have the same skill as your main.
And if you want to follow up with this logic, you really have 2 mains, not just a secondary. Just that the 2nd character isn't as versed in MU experience as your main is.