Host name common practice

[mmccaws2]

We have multiple AD forests host name problems.  Each forest has it's
independantly created DNS.  So NS1.AD1.here is SOA for AD1.here and
NS2.AD2.here is SOA for AD2.here.  AD1.here one-way trust AD2.here.
Administrator in AD2.here would like to rename server
hostname1.AD2.here but if hostname1.AD1.here is present the OS prevents
this.  The gotcha on this is if hostname1 is a unix box in AD1.here
then it is allowed.

Why is this happening if domains name servers are independant?

If duplicant host names are required on the two domains, what option
needs to be set in MS DNS.

Or is this even a DNS problem but only the symptoms appear as a DNS
problem?

Thanks for looking at it.

Mike