Your router will try to ARP for the 10.99.X.X out the interface with the 10.0.0.0/8 network assigned to it. If the router with the 10.99.X.X is connected to the 10.0.0.0/8 network and has ip proxy-arp enabled it should proxy the arp request and the packet will be sent out the interface. The router with the 10.99.X.X subnet will send the traffic.
Router 1 will ARP for 10.99.X.X..
router 2 (with IP proxy-arp enabled) will answer with it's arp address.
Router 1 will send the packet to router 2
ROuter 2 will forward the packet