As you have found a router can only run B,G,N at one time. There are what is called dual band routers than can actually run 2 at the same time but I think one of the radio's only runs at 5g so you would need nic cards that were also dual band.
From the radio side you will have no issues running 2 routers careful channel selection and SSID etc will make it all fine.
The harder part is how you connect both router to internet. You have 2 basic options. In both cases you connect one of your routers to the ISP. You then connect the second router to a LAN port on the first. Most likely you will use the WAN port on the second router but it depends on the device.
Just attach it and let it learn a address. The second router would then treat the first router as his ISP. This is messy because of the double NAT that will take place but it is easy to configure. Be sure to use a different ip block on the routers or they will get confused.
It will be a challenge if you need to have a device on one router talk to a device on the other.
Set the second router in bridge mode. This means your first router will treat the second as a dumb accesspoint. The second router will do all the wireless to the device and the first router will do all the IP stuff. This is a little more complex to get going at first since it is a uncommon config for a home router. It should be a little less complex to troubleshoot client problems and place fewer restrictions on the client to client traffic.
Not the best design but it is cheap and works.
I have a Linksys G router that's worked fine for me for my home network (three or four PCs sharing folders with each other and sharing high speed cable modem internet, three TiVOs accessing the net) for years. I've started streaming video to my WD Live Media Server and while it works for most of my streams my higher rate streams needing around 29mbps are freezing and failing.
Since my network adapters for those units are N, I thought I'd just buy an N router (D-Link DIR-655) and be done with it, as it's also backwards compatible with G (the TiVO units cannot use anything other than G adapters). However, I now read that running the N router in "compatibility mode" (I guess mixed mode as opposed to "green" something) essentially eliminates the N speed advantage.
Okay -- so could I run both routers together? One as a G mode router (for my G devices) and the other in strict N mode (for my media servers). They could be separate networks, I guess (although I'm not sure how to share the cable modem between them) and I could login to the appropriate one from the appropriate device.
Is this possible? Or advisable? Or is there something (hopefully easier?) else I could be doing?
TIA for any assistance.