Hi Patrick.
The trick of using more than one CD was negated when the developers increased the Elite design range and at the same time nerfed the CD. You will remember that until recently the range was 0-100,there was a start of season cap of 50 and a 5 star CD would generate 3.75 dp per race in his/her area of strength.
So even in a relatively short season of 17 races a CD could generate 17 x 3.75 = 63.75 dp (rounds to 64). Because of the cap 14 of those DP were wasted so the trick was to use more than one CD and this meant you had a higher number of dp at the start of the season than any competitor who was only using one CD.
Moving on to the current situation. A 5 star CD has been nerfed and now only generates 3 dp in strength, 2 dp in a standard attribute and 1 dp in weakness. So even in a 22 race season the most dp a single CD can generate is 22 x 3 = 66.
In addition to nerfing the CD the devs also increased Elite range to 0-200. I am not sure but I am guessing they simply doubled the original range which effectively halved the value of a single design point (it depends on your point of view). The start of season cap of 50 was either removed or also doubled, it doesn't matter which because if a CD can only achieve a maximum of 66 dp there is effectively no cap.
So the short answer to your question
I used this CD just 12 races then used another CD in the last 5 races to have 2 maxed out design. Is that strategy does not work anymore?
If you're looking to gain more
total design points than a competitor it doesn't work any more. If you are aiming to have a car that is balanced across more than one design area or want to maximise research then using more than one CD does still work but unless you're in one of the very top leagues I'm not convinced the benefit you get will compensate for the cost of doing it.
BTW: Using more than one CD will mean you will not be the strongest team in your CD strength areas if another manager has only used a single CD with the same strength. (if that makes sense) I believe this is what you are experiencing.