I think that perhaps we need to understand some better what they are attempting to accomplish. And perhaps we need to understand more about the topology and to be careful about how we use some terms.
If their situation were that router A connects to switch A where switch A uses VLAN 2 (only VLAN 2 or at least VLAN 2 as the native VLAN). And if switch A connects to switch B on an access port. And switch B uses VLAN 4 (only VLAN 4 or at least VLAN 4 as the native VLAN) and connects to router B. Then router A and router B could configure a single subnet (perhaps 10.10.10.0/24) and that subnet would operate in both VLAN 2 and 4.
But if the situation were that VLAN 2 and 4 both exist on the same switch and if the switch connects to a router via a trunk port, then the router can put a subnet on one VLAN or the other but it can not put the same subnet on both VLANs.
So perhaps you can find out a bit more and explain a bit more about the topology and about how they want it to work. Then we might be able to give better answers.
 after posting I read again the entire thread and I see that you mention a layer 3 switch. I could use an example with a layer 3 switch but I hope that my example with routers and switches explains the idea well enough.
We are pleased to announce availability of Beta software for 16.6.3. 16.6.3 will be the second rebuild on the 16.6 release train targeted towards Catalyst 9500/9400/9300/3850/3650 switching platforms. We are looking for early feedback from custome...