You are looking for ECMP - Equal Cost Multipath.
https://wiki.mikrotik.com/wiki/Manual:B ... interfaces
ECMP by default (route cache=on), will load balance per connection and not per packet.
By disabling route cache (in IP > Settings) then ECMP load balances per packet on all available paths.
But some other features may stop working with route cache disabled. It's a double edged sword really.