Increase sync interval and sync at the same time from 2 clients

hello, i was working on nextcloud for the last couple months

i need your help in increasing the sync speed here i mean the {interval/timing/} when the system check the files to sync them i have noticed that every 15 minutes the system is going to check and sync

but the question is can i make it 1 minutes?

the other question is when i use the same account on 2 clients and two persons would like to access the same file and make changes who is the first and who is the second ?

can we make kind of lock on the files that is beening accessed until this person is finished editing the file ? or make kind of notification system that the file have been changes while he is editing the same file?

i know that it looks hard a bit but it would give the system a geat push to make it flexible

The desktop sync client should sync every time when there are changes made to your files. Not just every 15 minutes.
What operating system are you using?

Maybe this app could help with the file locking?