Is it okay to have more than one user import data at the same time another user is importing data?
We have a bunch of segmented data to import split amongst multiple employees - is it okay if two employees are uploading at the same time?
Yes it is absolutely fine to import data simultaneously but it might take bit long time to complete it. If you want to perform multiple import simultaneously make sure you imports are not linked to each other.
I usually prefer to do it one by one after taking backup at each stages. You never know when mysql might crash with an error saying mysql has gone away, already exists, invalid data etc.
It should be okay if multiple users are not updating/uploaded same record type's same item at the same time.
Like for example if multiple users are importing a same contact record, that might cause an DB error possibly.