In Texas, the party’s controlled the numbers of booths and where these booths were located.
Neither wanted people from the other party using their booths, hence the separation.
Some counties contacted the parties (Dallas county, for example) and asked that they be allowed to allocate booths by precinct and projected voting numbers. Both parties declined.
The result was long lines at some Democrat heavy precincts. Sucks for them.
It went smoothly by all accounts that I have heard.
Interesting that the democrat voting screen warning against crossing over, or losing your privileges to voting in the after voting caucuses to determine voters at the state party convention.