Hello,
What you suggesting is entirely possible, as a matter of fact i have more 
then one index directory at one point, just remember to create appropriate 
client index association when creating new clients.
In the long run though you may just want to rebuild  the partition with 
more space including any growth.
As far as old client, you cloud just move index to new location and reset 
the index path ( Check the doc on moving indexes).
Is reducing the index size not an option for you ?
HTH

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