Hi ,
Did you mean that DHCP server in domain A, DNS server in domain B? Then you want DHCP in domain A write the A and PTR records regarding DomainB clients into DomainB DNS. Is it right?
Please feel free to let me know, if my understanding is wrong.
>>Would it be possible to add the DHCP server machine account in to the dnsupdateproxy group in the trusting domain?
The DNSUpdateProxy group can't add different domain's account due to it is a global group.

If the DomainB's Client get the IP address from DHCP role installed on the DomainA. Then it will get the DomainA's DNS server, when DHCP/CLIENT register the DomainB's Client into the DNS of DomainA, it found there is one secondary zone for DomainB which
cannot be reigstered.
It would be better to have a DHCP server for each domain and separate the domains into different subnets by routers.
Here is an similar thread discussed before, you might have a look:
One DHCP for two different domains
Best Regards,
Candy
Please remember to mark the replies as an answers if they help.
If you have feedback for TechNet Subscriber Support, contact
tnmff@microsoft.com