Hi

you can't connect more than pppoe on same mac address 

**your ISP make this rule**

try to divide on 2 interface using Switch  

Use one pppoe-client and route the additional IP to it.
You will need to manually assign it on the client RouterBoard.

Nick

Or you could run an autorouting protocol like BGP (with a private AS number) to cover cases like this.
Of course not linked to the full internet BGP!

Now I have a little experience with it I use it all the time and don't have to worry anymore
about what addresses and what subnets are located where.
BGP is really simple to setup.  But maybe it is something to remember when more situations
like this do appear.

I use only eBGP, on a closed network, and it is very easy to get going on MikroTik.
Only set an AS number from the private range, and set "redistribute connected", then add Peer
items for the directly connected routers with nexthop-choice=force-self ttl=1 and the peer AS and IP.
Optionally (recommended) add a simple output filter that accepts your network range and discards all others,
like:
add action=accept chain=filter-out prefix=10.0.0.0/8 prefix-length=8-32
add action=discard chain=filter-out

From then on, no routing worries anymore