See if they can support DD WRT. If they can, that is your best bet.
EDIT: I think what you want is technically called WDS. I'd search on the internet to see if those rotuers support it. Remember, WDS is also vendor specific. because you have routers from two different companies, they might support WDS but not with each other.
This is where DD WRT helps - its a third party open source firmware that includes WDS support. If they both support DD WRT, then you can install DD WRT onto them and they should be able to connect to each other via WDS. Remember that WDS will basically halve your wireless bandwidth.
EDIT #2: Another option is to have one DD WRT router connect to another using Wireless Client Mode. Wireless Client mode is another DD WRT feature, it makes your router connect to another network as if it were a network adapter itself. You can then share that network via the LAN ports of the second router. IE:
First Router <------------- Second Router Connected via Wireless Client Mode <---- devices connected to it via LAN cables. Can include another wireless router, for wireless connectivity on this side.
You will not be able to connect to the second router wirelessly, only via cable, unless you connect it via cable to a third router.
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