I export prefixes to this bgp group of 2 neighbours with the same AS with multipath and load balanced per packet. The upstream is balancing between them but what I really need is to do the same with the downstream; I need to export just the half of prefixes to one neighbor and the othe half to other neighbor.
Can you share your rough topology? It's not quite clear where do you want to load balance.
a) You can advertised all the routes to both neighbor and then apply a policy in which you advertised higher med for half of the routes to ISP-R1 and for other half to ISP-R2. (In case of EBGP peering)
b) Same you can do with as-path prepend.
c) you can use origin code as well
d) you can also use Local preference in case of IBGP neighborship.