Это будет prerouting или тоже forward?
/ip firewall mangle add action=mark-connection chain=prerouting \
comment="Connmark in from ISP2" connection-mark=no-mark in-interface=pppoe-out1 \
new-connection-mark=conn_isp2 passthrough=no
/ip firewall mangle add action=mark-routing chain=prerouting \
comment="Routemark transit out via ISP2" connection-mark=conn_isp2 \
dst-address-type=!local in-interface-list=!WAN new-routing-mark=to_isp2 passthrough=no