Apologies in advance for the botched terminology. I have read the Server Fault Subnet Wiki but this is more of an ISP question.

I currently have a /27 block of public IPs. I use give my router the first address in this pool and then use 1-to-1 NAT for all the servers behind the firewall, so that they each get their own public IP.

The router/firewall is currently using (actual addresses removed to protect the guilty):

IP Address:  XXX.XXX.XXX.164
Subnet mask: 255.255.255.224
Gateway:     XXX.XXX.XXX.161

What I would like to do is break out my subnet into two separate /28 subnets. And do this in a way that is transparent to the ISP (i.e., they see me as continuing to operate a single /27).

Currently, my topology looks like:

     ISP
      |
[Router/Firewall]
      |
  [Managed Ethernet Switch]
  /       \         \
[Server1] [Server2] [Server3] (etc)

Instead, I would like it to look like:

       ISP
        |
    [Switch]
    /      \
[Router1] [Router2]
  |    |    |   |
[S1] [S2] [S3] [S4] (etc)

As you can see, this would partition me into two separate networks.

I'm struggling with what the correct IP settings would be on Router1 and Router2.

Here's what I have right now:

              Router1              Router2
IP Address:   XXX.XXX.XXX.164      XXX.XXX.XXX.180
Subnet mask:  255.255.255.240      255.255.255.240
Gateway:      XXX.XXX.XXX.161      XXX.XXX.XXX.161

Note that normally you would expect Router2 to have a gateway of .177, but I'm trying to get them both to use the gateway originally given to me by the ISP.

Is subnetting like this in fact possible, or am I completely botching the most basic concepts?