The idea is good, but the execution I dont think would be possible, due to the size of the LF antenna on a flex…
Due to the frequency
@Satur9 described this issue in another thread.
Basically imagine the 13.56MHz antenna is i think 22m long or a fraction of ie. 1/4 1/2 so 5.5m 11m etc.
BUT
when you talk LF 125khz the antenna length is 2.4km so yeah you can fraction that, but it is NOT going to fit in a flex form factor… well not one that will easily fit into you.
Let me find @Satur9 post on it, he would have explained it far better than I did…wait one
UPDATE
HERE it is.
That lad really knows his shit.
THIS is a real rough guess no mathematical working out what so ever, but my GUESS would be a Flex xEM would be a variation of these dimensions 12mm x 70mm x 1mm… I shouldn’t even leave this in because it is pure plucking out of the arse speculation eye-ometer guesswork
I would ASSUME though, a 2in1 xSeries HF LF xLED would be ?possible? only because @amal hasn’t shot me down when I have mentioned it previously… I guess you just take out the NT and ATA chips from a NExT and throw in a couple of LEDs. Simple 
