This article is from the RSC/UKSC Cricket FAQ, by Ganesh with numerous contributions by others.
Quoting from a ACB press release:
"There has never been a hard and fast rule to cater for
this situation, but to avoid any possible misunderstandings
in future, from now on if more than one player makes their
debuts in the same match, the allocation of cap numbers will
be done alphabetically and not by batting order."
This press release was released after a special excemtion was
made for Micheal Slater to allow him to continue done after a spec