I see this not possible without manupulation of the outgoing traffic.
Do you have bgp table from both ISP's ?
If yes, you will need to do some policy routing to state that you will prefer ISP_A to reach that all internet (as it is 5 Mbit) and prefer ISP_B (as far as ISP_B link is up)to reach ASxxx, ASyyy, etc.. This is not very flexible solution and will need fine tuning.
HTH,
Cisand