Question: Okay, suppose that I have 2 computers: `Computer A` and `Computer B`, connected with this setup:

```
A->switch->router->internet
          /
B->router/

```

and the closest router to `computer B` is configured to block connections from `Computer A`s switch.

If `computer A` is desperately trying to attack `computer B`, sniffing packets, MiTM attacks, etc., how much bigger/smaller attack surface does it have than the usual

```
Computer A ->router->internet
           /
Computer B/

```

Answer: 
The basic rule is that on the same ethernet network, IP address spoofing is possible, usually by performing ARP spoofing.

In your second scenario, computer B is completely vulnerable to computer A. Well, unless there is protection at a higher layer, e.g. using SSL.

In this scenario, computer B is NOT vulnerable to computer A:

```
Computer A --> Router -\
                        >- Internet
Computer B --> Router -/

```

Of course, this assumes that both the routers are trusted.

In your first scenario, computer B is still vulnerable. Although it is on a separate ethernet network, the problem is that B's router is on the same ethernet network as A. So although A can't directly spoof B, it can spoof B's router, which still allows A to MITM.
Computer A needs to control a device which handles Computer B's traffic on order to perform a MiTM attack. In your top scenario there's a bigger attack surface because there are 2 routers that computer A can attempt to compromise.