Well, I'm using CPU fan header for S12A, minimum speed I can set is 21%, which corresponds with 360RPM. If I'm adding heatsink fan, I will use PWM splitter and run all fans from CPU header. So basically I need fan with similar duty cycle/RPM response curve.
If your FanXpert2 fan control is similar to mine (it should, mine is AI SUITE III but FanXpert2), then i think NF-A15 PWM is a good match, the control range is almost the same 15% to 17% minimums respectively. Still i have my doubts being totally the same since you are on 360rpms while im at 250-275 rpms on idle, regardless, the fan curve is very similar on both fans so even if there is a bump in minimum speeds should be both the same in terms of PWM SIGNAL.
Personally i would run the NF-A15 PWM as the rpm signal connector on the splitter, as it has a minimum of 17%, this would bump the NF-S12A from 15% to 17%, but the difference should be negligible, but this will ensure that both fans spin always. But feel free to test both fans on your hardware/software as not all samples of fans are 100% identical, so its best to test once you have it and you decide which of the two runs on the RPM signal connector on the splitter (one has 4 cables the other has 3).
Now on a lot of ASUS motherboards, you have a CPU_FAN and CPU_OPT, the last is just a mirror from the first, its not controllable, but it has an advantage over running it with a splitter, that CPU_OPT does read the RPMS, so you will have display on you AI SUITE of both fans RPMS, i cant say for sure as i don't know which motherboards you own, but check if you have the CPU_OPT header, and if you do, use that instead of the splitter.
I'm not sure who can we blame CHA fan headers limitation, did you actually try them with voltmeter? I think I'll try that, I'm still not convinced that CHA fans are just voltage controlled 4pin headers.. are you really sure that they are really not just PWM headers.
All i can tell you is that i have done a lot of testing on my Asus Maximus VI Gene, and read a lot of other people tests, but i havent used other asus motherboards with fanxpert2 to know for sure if this is how it is on all, its just my personal assumption. I also didnt bother testing all CHA_FANs, maybe one isnt voltage controlled but a true PWM header. But if you do run voltmeter test let me know, im interested on this, as ASUS market their motherboards as full PWM headers, just not my experience thats true, again i can be wrong, so please let me know if you do find it. Another thing you can do if you feel like it, is enter the bios and go into fan control and change the Q-FAN from AUTO to ADVANCED, this will make the CPU_FAN switch from being a true pwm header to voltage control header, run fanXpert2 on it like that see if the curve is the same as what you are getting on the CHA_FAN headers. If you find anything post it, im interested on this, as this was one of the reasons im not buying ASUS motherboards.
Scythe Kama PWM
While i love scythe fans, specially slipstreams and kamaflex pwms, these were the first pwm fans that were done right, even before noctua, for someone looking for a very low dropping quiet fan. But in terms of PWM they are a little (not much) wierd, in some cases their PWM fans drop lower on voltage control then on PWM signal (this is not the case on Noctuas). Lawrance from SPCR has recently started dual testing the fans on the reviews, so you can see some of this cases there also.
From Scythe Ashura CPU Cooler
From Scythe GlideStream, Slip Stream XT, and Grand Flex Fans
page 3 n 4
As you can see from the three examples (there are more), some scythe "PWM" fans drop lower on voltage control fan than on true PWM control, but this is how these fans are. There are other Scythe PWM fans that are not like that, they are like noctua that drop much lower on PWM than on voltage control, the only way to know is to check reviews like SPCR or do your own testing. This is the main reason i decided to keep my asus motherboard for pure fan testing, as not all fans are the same.
I guess you're right, CHA 4pin headers are in fact NOT PWM headers.
Don't accept what im saying is right, test it and draw your own conclusion. But i do recommend you to read ASUS Z87-Deluxe fake 4-pin headers & other fan control info
, its a good discussion on the subject and the experience of some users.