Your initial idea was not bad at all. What you can do is store the unwanted partitions with their local indexes in separate tablespaces. Use rman for the cloning but use the SKIP TABLESPACE option to not clone the tablespaces with the unwanted partitions. (assuming online backup)
After the clone, the skipped tablespaces have datafiles with status RECOVER.
see RMAN DUPLICATE DATABASE: Options
In the end you just drop the unwanted partitions. To be able to do that you first have to get rid of things like constraints and indexes that need to be re-created later on. This worked in 10gR2. Make sure that you don't drop the last partition of a table, in that case drop the table.
It is a bit of work but certainly possible. If the difference in Volume is huge or there are lots of copies, it might be worth spending some time for it.
To use multiple disks for IO, your best bet is to use RAID, preferably hardware RAID, giving both increased reliability and performance both.
Trying to do it by hand is horrifyingly bad; you're unlikely to get a good performance balance, that performance balance probably, won't survive table growth, and a single disk failure destroys your data - you're effectively suggesting RAID 0 (striping) with all the disadvantages, but without most of the benefits.
If you insist on the mindbogglingly crazy idea you're looking at, then MS SQL Server Enterprise Edition (and likely other products) has partitioned tables, and that would allow you to have one table with partitions on different filegroups, and then you could put the different files of the filegroups on different partitions. You'd control proportions by the choice/control of the data the partitioning is done on.
The budget crazy idea is multiple tables to store different percentages of the data (by hand partitioning), and then UNION them into a view.
Again, don't do this - use RAID*. Heck, even use software RAID if you have too low a budget for hardware RAID!
* RAID 1, 10, 5, 50, 6, and 60 are approved variants for databases. Do NOT use RAID 0.
Best Answer
Few uses for the tablespaces: