I recently started colocating at a datacenter, and they provided me with a /27 subnet, let's say
192.0.2.112/27 , this translates to usable IP's .96 to .127.
Because of a requirement in my network, I need to have two subnets, so I can assign each /28 subnet to its own router (I have two routers in my rack).
In this case I will have:
192.0.2.96/28 on my primary router , gateway 192.0.2.96
192.0.2.112/28 on my secondary router, same gateway.
I understand I will have one less usable public IP because of the broadcast address and the network address, but other than that, are there any issues if I do that? Do I need to notify the colo ISP or can I do whatever I want with my subnet?
If your provider is routing to you, then yes, you can split it. If it is bridged then you cannot.
Based on your description it looks like you're bridged.
If you want routed subnets you need more subnets assigned by your provider. You always need (at least) one bridged network. Then any additional subnets you have been assigned can be split any way you like.
You don't need multiple routers, you just need multiple interfaces* on a single router.
* Multiple VLAN interfaces on a single physical link works too.