Unfortunately, this isn't supported to my knowledge. You can turn up lots of documentation on 'ISDN NFAS' on CCO, which is the technology used to control B channels on multiple PRIs from one D channel, and to support an alternate backup D channel if the first one dies. However, this only works within one gateway, not between multiple gateways.
http://www.cisco.com/en/US/tech/tk801/tk379/technologies_configuration_example09186a00800a6bf2.shtml
This sort of thing can be done using SS7 signaling external to both gateways, but this is a rare configuration and usually limited to service providers. It's really unlikely that you can do this.
You could move them both to one gateway and make this work. However, if this is to be configured with CallManager, note that CallManager has no idea how to handle NFAS PRIs so you'll need to use H.323 signaling instead of MGCP. If you're already planning to use H.323 or another technology where the router handles all q.931 signaling, you're fine.
