question. Multiple domains in the dns section of a forward zone file??

I am having a tough time finding references for this in online searches.

My scenario:
My domain is mydomain.com
$ORIGIN or @ indicate mydomain.com
vps.mydomain.com is authorative
second and third domain (or more) are on a different network and have a different domain name. otherdomain.com

I have assumed that forward zone would contain lines (not limited to) these:

$ORIGIN mydomain.com.
@ IN SOA vps.mydomain.com. admin.mydomain.com. (
...
...
;NS area
@ IN NS vps.mydomain.com.
@ IN NS a.ns.otherdomain.com. ;there is a "NS." before otherdomain.com.
@ IN NS b.ns.otherdomain.com.

;A records
@ IN A 10.10.10.11; my made up IP
localhost IN A 127.0.0.1
vps IN A 10.10.10.11
mail IN A 10.10.10.11
a IN A 123.123.123.124; IP of a.ns.otherdomain.com
b IN A 234.234.234.235; IP of b.ns.otherdomain.com

;MX record(s)
@ IN MX10 mydomain.com.

;TXT record
@ IN TXT "not sure what to put here, or if needed"

;Aliases
sftp IN CNAME mydomain.com.
www IN CNAME mydomain.com.

From what I have read so far, my ;NS area is wrong.
I believe @ represents $ORIGIN, or mydomain.com, but is used incorrectly for otherdomain.com servers. ??
Am I correct in thinking that a space instead of @ simply means mydomain.com anyways?
I have been reading a lot, but am unsure what would be in front of IN NS of a.ns.otherdomain.com, and b.ns.otherdomain.com.
Puzzling, but fun so far.
Thanks for reading all this.