On a CCR2004,I have setup 2 WAN (2 different lines/ISPs) for IPV6 failover using BGP. Failover is working fine but I know would like to fully use these connections. Is this possible using internet rules? Or should I use 2 routers each connected to a different ISP?

The questions a good start but a bit broad, are you able to give us an idea on the end functionality, do you want specific services to only go out one ip, do you just want it per connection, do you want to bond the interfaces and improve the throughput etc.
There are many different ways to set this up depending on the behaviour you are after, remember that all the configuration outside of BGP will control how the outgoing packets will be
forwarded, you will need to set up your BGP correctly with your peer's if you would like to influence how incoming traffic behaves

The questions a good start but a bit broad, are you able to give us an idea on the end functionality, do you want specific services to only go out one ip, do you just want it per connection, do you want to bond the interfaces and improve the throughput etc.
There are many different ways to set this up depending on the behaviour you are after, remember that all the configuration outside of BGP will control how the outgoing packets will be
forwarded, you will need to set up your BGP correctly with your peer's if you would like to influence how incoming traffic behaves

At some point I may want to bond them. But at first i would like to pass the traffic on the different, each wan would have this ow IP prefix. Where should I read to the different way to setup the different wan anyway? Is there any documentation of IPv6 + routing rules usage?

again depending on what you want to achieve this can be done through the route table or it can be done through the use of mangle, unfortunately you will not be able to use the mangle method on ipv6 traffic on router OS6 and you would need to use ROS7 and its new way of policy routing.
Other than that routing is just routing so if you have done this with ipv4 you should not have any issues with this

First link will be to all the cool routing stuff Mikrotik have added to the routing 'engine' as well as letting us know what is working and what is no https://help.mikrotik.com/docs/display/ ... l+Overview

second link will be how policy routing works on router OS 7 as there has been some changes between 6 and 7: https://help.mikrotik.com/docs/display/ ... cy+Routing

Other than that routing is just routing so if you have done this with ipv4 you should not have any issues with this

Right, but not sure that performances are the same. It seems that everything in IPV4 is offloaded but nit with IPv6 . At least on the CCR2004. But maybe I'm wrong?

Looking over the documentation it looks like everything should be set up for ipv6 at this layer though? Looking at the routing overview the functionality is there: https://help.mikrotik.com/docs/display/ ... l+Overview
And if we look at the l3 hardware offloading it looks like ipv6 has been implemented in some way although it looks like it uses significantly more ram so keep that in mind.