If any of you know of a better solution please let me know but here goes...

I have found that even with two cards installed if you have one scheduled recording immediately following another it will ignore any soft-over-recording (record additional minutes at the beginning and end if no conflict) and immediately use the same card for the next recording.
The problem with this is that you can often miss the start or end of your program when the switch occurs. Meanwhile the second card just sits there twiddling its thumbs doing nothing. I have looked through the settings and cannot see any setting telling Myth to utilise both cards where possible to over record and so I came up with an alternative.

I have made all recordings HARD-over-record by one minute at the end. By doing this we cause conflicts with recordings immediately following one another but we have a second card. The second recording is assigned to the second card and then both recordings are allowed to SOFT-over-record. If the extra HARD-over-record minute causes an unrecoverable conflict then you can go back in and manually edit one recording but on the whole I think this will solve more problems than it will cause.

Let me know what you think,
Glen