Can't remember the exact episode dates, will need to check.
In any event, even if the will in Shyam's possession ("Shyam's Will") is executed after the will in Akash's favour ("Akash's Will"), it will not have an automatic superseding effect.
For Shyam's Will to supersede Akash's Will, there has to be an express clause in Shyam's Will revoking all prior wills and codicils OR some other written document from ASR stating that he is revoking it. Only ASR has the right to alter / vary / destroy his will - he cannot pass on this power to anyone.
However, since Shyam's Will is likely to be ASR's first will, there would not have been an express revocation clause in that will and when Shyam revised the will, he didn't know about Akash's Will so he would have only replaced the executor/beneficiary clause. Therefore, even if Shyam plays the Will trump during ASR's absence, the whole thing may just blow up in his face. Because Akash will have the original will and Khushi will know that Shyam fooled her into signing the new will by fraud.
But isn't there a default clause in every will to the effect that all other wills/testaments made are null and void?