Sub dividing the block of IPs in different backbones

I have not used BGP myself. What I have read about it suggests that what you want to do is possible in theory but not in practice, as the policy is usually not to advertise AS numbers with prefixes that long, so as to avoid filling up routing tables with lots of small entries.

The usual solution to this would be to get a second IP range from the one ISP that the traffic is to travel on and to use that for the single-backbone traffic. That distinct IP range would be as large or as small as needed, since it would be part of the IP range allocated to that ISP and so that ISP would be aggregating the IP into a larger block for BGP advertising purposes.