May I suggest that there may be a more thoughtful way of running with a simple 2 DNS solution.
If the purpose of multiple DNS is to create fall-over support should one fail, I would suggest that adding two DNS servers of the same ISP to be unwise, as everyone using that ISP will use the fall-over of the 2nd DNS for that ISP.
If ISP(a) has 2 DNS servers and server 1 crashes due to heavy workload then I suspect the 2nd will get all DNS requests from the first due to fall-over and similarly crash, perhaps in the meantime (until 4.5 comes out) set DNS 1 to be your main ISP DNS and set DNS2 to point to a second ISP or openDNS, given that openDNS is geared up to handle a significant amount of traffic, one or two sites more (no matter how big) probably won't be a big strain for them (Unless openDNS is blocked from your ISP).
In 4.3, you can only have primary and secondary DNS entries http://wiki.mikrotik.com/wiki/File:Pppoe-dns-server.JPG
, it seems that with the DHCP server option you can add more via winbox (I didn't know that option existed!!!), but only for IP pools that are actively being served by your MT DHCP server. Sorry it doesn't work in your code