With IPv6 you normally can’t choose route because end devices have public (routable) IPv6 addresses, so no NAT (which actually enables choosing route in IPv4). The only thing Mikrotik router can do is to choose route depending on src-address. The way you can affect choice of route is by making LAN devices to have “the right” IPv6 address. Which is not an easy task itself, but possibilities depend on what LAN infrastructure devices you have. If you have a proper DHCPv6 server running and devices are not android, then you can configure static IPv6 leases. Or you can run two LANs (or VLANs) with different IPv6 subnets…
ROS doesn’t provide proper DHCPv6 server if you’re thinking about that. Other options are open, which one fits depends on your network topology and what is possible to do. If you can’t figure it out yourself, hire a Miktotik consultant.
Are you saying mikrotik should automatically use the right route according to src address? Assigning different ipv6 address prefix to different lans is easy and they pick up from router advertisement, but then they dont work..