I only know a way to achieve this if the two nets which needs to be added are connected by an resistor, cap or so:
assuming your nets look like this:
(I/O1) --net1---Resistor---net2---(I/O2)
(I/O3) --net3---Resistor---net4---(I/O4)
you can create XNETS for I/O1 --- I/O2 and I/O3---I/O4.
You need to assign models for the passive parts (resistors or caps) which are connecting the nets. Then create an match group and add both Xnets. Yust add the min/max etch rule ore relative etch rule.
Tobs