Question : Problem: Combining two physical interfaces into one on Cisco 2800

I have two subnets on separate segments each with an interface on the Cisco 2800.  Subnet A is 10.1.12.x and Subnet B is 10.1.13.x both using a 255.255.255.0 mask.   I only have subnet A physically connected to half of my IDF closets but subnet B resides in all of them.  Can I combine the two separate physical interfaces on the Cisco in such a way that I would be able to attach a subnet A device (10.1.12.x) to a closet that doesn't have a physical 10.1.12.x port?  I am trying to find a way around having to run additional fiber and purchase additional switches for the closets that do not have both subnets in them.  I've been told it's possible to accomplish this at the router level (the 2800) but not given any explanation as to how.  

Answer : Problem: Combining two physical interfaces into one on Cisco 2800

Sounds like someone has suggested that the router be used to bridge the two networks, but then the two networks will no longer be separate.
What you want to do should be supported by your switches by creating Virtual LANs (VLANs) , being able to assign ports to a specific VLAN and have the links between switches support VLAN tagging so that you can have traffic for multiple networks through the same link but still be separate.

What kind of switches do you have (manufacture and model) and how are your IDF closets connected back to a central distribution?