is it possible to have a Mtik board use one radius server and then if something happens, be it the first one falls over or is taken down for maint, use a second one
Yes. Just add a second entry in the /radius section. If the first radius server does not respond, the second will be used.
This does not mean if the first server sends a Access-Reject it will check the second server. Then it does not use the second server.