I am not sure why you are talking about a "second" /30. What I mean is ask for 2 subnets from ISP: one /30 for the WAN, and one /29 pointed at your WAN address on their router.
For example, pretend they give you 10.0.155.164/30 and 10.0.155.168/29. The /30 would be for the WAN, with .165 on their router and .166 on yours. Then they would route 10.0.155.168/29 to 10.0.155.166.
In RouterOS speak, it would look like this on their router:
/ip address add address=10.0.155.165/30 interface=ibm
/ip route add dst-address=10.0.155.168/29 gateway=10.0.155.166
...and then on your RB1, you would do this:
/ip address add address=10.0.155.166/30 interface=WAN
/ip address add address=10.0.155.169/29 interface=RB2
/ip route add dst-address=0.0.0.0/0 gateway=10.0.155.165
...and finally, on RB2, you would do this:
/ip address add address=10.0.155.170/29 interface=RB1
/ip route add dst-address=0.0.0.0/0 gateway=10.0.155.169
Now, if you are talking about "second" /30 because you are thinking about splitting your /29 into 2 /30s, you can do this, but because your provider is treating it as a /29 on their side, you will have to treat one half of the /29 (the one with their gateway address in it) still as a /29, create an address in the other half of the /29 as a /30, and enable proxy-arp on your WAN interface. On RB1:
/ip address add address=10.0.155.173/29 interface=WAN
/ip address add address=10.0.155.169/30 interface=RB2
/ip route add dst-address=0.0.0.0/0 gateway=10.0.155.174
/interface ethernet set WAN arp=proxy-arp
...and on RB2:
/ip address add address=10.0.155.170/30 interface=RB1
/ip route add dst-address=0.0.0.0/0 gateway=10.0.155.169
...but this is a very wasteful and inefficient use of the IPs in such a small subnet. The only remaining usable IP address after this will be 10.0.155.172, and only if you bridge it directly to the ISP and use .174 as your gateway for that device.
If there will not be any other devices except for RB1 and RB2 (or of there are others, you will use NAT for them), and your ISP won't give you a /29 and a /30, you could try to ask them to split the /29 into 2 /30s on their side and use one as the WAN subnet and route the other /30 to you in the same way I described for the /29. You would set that up exactly the same way that I described in the example with the routed /29.
You may want to go pick up a good book on the subject of networking, IP routing, and ethernet.
-- Nathan