Oooch, what a genius idea 
I guess you have to do this on all the devices in the LAN remaining hear/speak over the standard 21027 isn’t? Why not tweaking the firewall only on the device(s) running multiple ST instances this way:
iptables -t nat -A OUTPUT -p udp --dport 21026 -j DNAT --to-destination :21027
iptables -t nat -A INPUT -p udp --dport 21027 -j SNAT --to-destination :21026
?