Is there any reason why we shouldn't run these too database servers on the one machine?
There shouldn't be any interference between the two servers that would break things. For example, you need to install both of them on the same machine when running Blackberry Enterprise Server for Domino.
But as lg says, performance may be a concern depending on what you need from them.
As above performance, also as sql is licenced by the socket you may to ensure that socket is used purely for sql as much as possible. Of course if you've got more than one cpu and affinity set for both apps (don't if that is possible for domino but I guess it is) then this is less relevant.
Performance, anyway it depends by hardware and by the use of both servers.