Septet session

Things here have been rather quiet of late, for which I apologise.  However, this evening I have a session of string septets to go to.  The programme consists of the Strauss Metamorphosen as well as septets by Martinu and the leader of the group.

On first inspection, the Martinu doesn't appear to be too tricky, but, having played his mixed nonet on many an occasion, this may be misleading.  I'll write again if there are any tricky bits!  Hmm.  -1 for observation - the part says "Sextet" at the top - is it one of those optional double bass / 2nd cello things?  [Edit - turns out that it is a sextet with an optional bass part; fortunately for my sanity, the bass part is not just a follow-the-cello thing, but adds a distinct texture to the piece.]

As for the other piece...  Anyone got any suggestions about how to play passages in running octaves?  I don't think my fingers will stretch to double-stopping this low down on the bass!  Sample below...

[Edit - the composer says that he wants the player to play the bottom line if the bass is extended, or a five string, and the top line otherwise...]