I use Tailscale, and you can set up a subnet router for that particular use case.

This is an unusual home network topology. This would be something like a distribution layer, so a and b would be something like a multilayer switch. This would only really be for something like load balancing or redundancy, and isn’t really a home network topology. I guess the question is really, what are you trying to do? If the answer is just dick around with network stuff, a switch is what you need. If you are trying to set up a normal home lab network, I would drop them as other commenters have shown.