What you actually want is to tell RouterOS to send queries for specific domain(s) (your local one(s)) to DNS server in location A, while using resolver(s) from /ip dns for everything else. It’s basic feature supported by probably every other DNS software, but unfortunately not by resolver in RouterOS.
Current sort-of-usable better-than-nothing solution for RouterOS is old L7 hack, where you use L7 matcher on client’s udp packets with queries and redirect them to the right server using dstnat. It bypasses cache on router, doesn’t work for tcp queries, can’t be used with IPv6, but if you can live without these things, it’s usable.
It was requested several times in this forum over the years, but the only response from some MikroTik’s employee I remember was something like “just set up a server with bind”, which is ridiculous solution. It’s huge overkill if resolver in RouterOS was otherwise enough for user, and also because this feature is mainly attractive for smaller users without proper DNS infrastructure. If you set up server with bind, you might as well setup proper zone transfers and everything.
Try to send feature request to MikroTik’s support. I plan to do it myself, because it’s crazy that this feature is still not there, but I don’t know when I’ll get to it (I’d like to make it really convincing). But the more people ask for it, the better.