If I'm updating v10 membership, event scheduling and record keeping only and copying the data to a USB flash drive, Will I be able to use the flash drive of v10 updates to maintain the membership, event scheduling, and record keeping only in v11.5?
NO .. . the database is quite different in the two versions. Thats why you should upgrade everything. Its best to network any two computers and use one as the 'server'. Otherwise you will start having 'sync' issues if one's membership information is updated, the other isn't, and you backup from the older data to the new, overwriting the newer data.
It just ISN'T a good idea to run two separate versions trying to maintain current data on both. I would keep the v10 setup just for historical information if necessary, and move EVERYTHING to v11.5, which does give you a better set of data screens, as well as membeship, attendance and contributions information in one place on a profile.
If you're worried about people having access to data they aren't supposed to, you can setup the password & user system to make sure they don't have that access.
Could you tell us how you're planning to use the system? Both computers in one office, one at home?
If you're planning on keep the accounting data on a separate computer, then I would just keep the accounting NOT the contributions. The contribution data does NOT need to reside on the same computer as the contributions. You can post contributions to a file, which then can be 'sneaker netted' to the accounting computer to import via a USB drive.
However, more details on what you're plans are would help.