I would recommand you to use multuple subdomains.
192.168.42.x is mylan1.com
and 192.168.142.x is mylan2.com

So if the laptops in those different subdomains request the _NON_-FQDN Instrument1, they get different answers, based on the two A-Entrys:
Instrument1.mylan1.com -> 192.168.42.254
Instrument1.mylan2.com -> 192.168.142.254