This is not valid, no. It will probably return the 1.1.1.1 as it would get returned by the first server that resolves but the "correct" value should be whichever nameserver is registered as the primary by the SOA record. But, what gets returned may depend on which version of nameserver software is run. See below, it will just forward it on to the next nameserver, and 1.1.1.1 will never get shown.
Now it IS correct to list other NS records in your zone, but the other NS should be nameservers with identical zones. You can point your subdomains to alternate nameservers however.
So example.com by your registrar points to ns1.example.com and ns2.example.com with 1.2.3.4 and 2.3.4.5 IPs from the registrar nameserver (glue).
You would then have a zone with the SOA choosing ns1 or ns2 as the primary, and then you would have two NS records pointing to ns1.example.com and ns2.example.com with the A records for that domain (and mx, txt, cname, etc).
Both NS1 and NS2.example.com should have identical zones and they should replicate to each other automatically.
Now it IS valid, to take test.example.com and point that to ns1.somethingelse.com and ns2.somethingelse.com but NO A records for test.example.com on ns1 and ns2.example.com's nameservers, except ns1 and ns2.example.com should automatically send the IPs for ns1 and ns2.somethingelse.com if it has glue. (It won't if the TLD is different, ie .com and .org)
I hope that all made sense, I can clarify more if any of it is confusing.
Here's a test of what happens:
On shadowrpg.net's nameserver:
$ttl 38400
@ IN SOA shell2.reganw.com. root.shell2.reganw.com. (
1298345653
10800
3600
604800
38400 )
@ IN NS shell2.reganw.com.
test.shadowrpg.net. IN A 127.0.0.1
test.shadowrpg.net. IN NS saber.reganw.com.
On test.shadowrpg.net's nameserver (saber):
$ttl 38400
@ IN SOA saber.reganw.com. root.shell2.reganw.com. (
1298345653
10800
3600
604800
38400 )
@ IN NS saber.reganw.com.
test.shadowrpg.net. IN A 127.0.0.2
test.shadowrpg.net. IN NS saber.reganw.com.
The first result, is with saber not configured, to show the referral.
[regan@gamma ~]$ dig +trace test.shadowrpg.net
; <<>> DiG 9.7.3-P1-RedHat-9.7.3-2.P1.fc13 <<>> +trace test.shadowrpg.net
;; global options: +cmd
. 518400 IN NS G.ROOT-SERVERS.NET.
. 518400 IN NS C.ROOT-SERVERS.NET. (snip)
;; Received 512 bytes from 127.0.0.1#53(127.0.0.1) in 2 ms
net. 172800 IN NS a.gtld-servers.net.
net. 172800 IN NS m.gtld-servers.net. (snip)
;; Received 493 bytes from 199.7.83.42#53(199.7.83.42) in 55 ms
shadowrpg.net. 172800 IN NS ns1.reganw.com.
shadowrpg.net. 172800 IN NS ns2.reganw.com.
shadowrpg.net. 172800 IN NS ns3.reganw.com.
;; Received 232 bytes from 192.35.51.30#53(192.35.51.30) in 54 ms
test.shadowrpg.net. 38400 IN NS saber.reganw.com.
;; Received 110 bytes from 209.161.6.3#53(209.161.6.3) in 2123 ms
test.shadowrpg.net. 38400 IN NS saber.reganw.com.
;; BAD (HORIZONTAL) REFERRAL
;; Received 110 bytes from 173.45.238.245#53(173.45.238.245) in 81 ms
test.shadowrpg.net. 38400 IN NS saber.reganw.com.
;; BAD (HORIZONTAL) REFERRAL
;; Received 110 bytes from 173.45.238.245#53(173.45.238.245) in 82 ms
test.shadowrpg.net. 38400 IN NS saber.reganw.com.
;; BAD (HORIZONTAL) REFERRAL
;; Received 110 bytes from 173.45.238.245#53(173.45.238.245) in 112 ms
And with saber configured:
^C[regan@gamma ~]$ dig +trace test.shadowrpg.net
; <<>> DiG 9.7.3-P1-RedHat-9.7.3-2.P1.fc13 <<>> +trace test.shadowrpg.net
;; global options: +cmd
. 518400 IN NS L.ROOT-SERVERS.NET.
. 518400 IN NS J.ROOT-SERVERS.NET. (snip)
;; Received 512 bytes from 127.0.0.1#53(127.0.0.1) in 2 ms
net. 172800 IN NS m.gtld-servers.net.
net. 172800 IN NS a.gtld-servers.net. (snip)
;; Received 493 bytes from 198.41.0.4#53(198.41.0.4) in 129 ms
shadowrpg.net. 172800 IN NS ns1.reganw.com.
shadowrpg.net. 172800 IN NS ns2.reganw.com.
shadowrpg.net. 172800 IN NS ns3.reganw.com.
;; Received 232 bytes from 192.12.94.30#53(192.12.94.30) in 165 ms
test.shadowrpg.net. 38400 IN NS saber.reganw.com.
;; Received 110 bytes from 209.161.6.3#53(209.161.6.3) in 38 ms
test.shadowrpg.net. 38400 IN A 127.0.0.2
test.shadowrpg.net. 38400 IN NS saber.reganw.com.
;; Received 126 bytes from 173.45.238.245#53(173.45.238.245) in 80 ms
[regan@gamma ~]$
So once you add an NS record, it refers it on, and it ignores any query to itself. Now if you add an A record for the nameserver, it will pass with the NS record as a glue (so the resolver doesn't have to then look up the IP for the new nameserver).