I think those rules actually don't work, but it depends on your exact network topology.
First of all, DHCP uses anycast, not assigned addresses.
Let's analyze 2 use cases assuming the following topology:
clients A ---- Router A ----------Router B ---- clients B
1-st use case: You need all clients on the same broadcast domain. This is needed e.g. to be able to browse workgroups on windows networks, if you don't want to set up a wins server.
In that case, you will need to set up 2 bridges, so that broadcasts from clients A could reach clients B (which I unterstand you did).
But the bridge traffic is not affected by forward rules in the router, only by bridge filter rules, so blocking udp ports 67 and 68 from the local port of each bridge to the link port has to be done by bridge filter rules.
Actually you need to do this only on one router, but block the specific traffic in both directions.
For these please check http://wiki.mikrotik.com/wiki/Manual:In ... ket_Filter
2-nd use case: You are happy with just inter-networking between those segments, and then you sub-netting your internal network in 2 segments, each serviced by a DHCP server. In this case, you only need to route between those 2 LAN segments via the link (which can be part of a different network, or a third sub-net). Because anycasts and broadcasts can not traverse routers, there is no conflict between the 2 DHCP servers.
Now you need to decide what you need to do.
Torturing CCR1009-7G-1C-1S+, RB450G, RB750GL, RB951G-2HnD, RB960PGS, RB260GSP, OmniTIK 5HnD and NetMetal 922UAGS-5HPacD + R11e-5HnD in my home network.