Opening two of the same wallet (tested in v0.18.0 of bitcoin-qt 64 bit) will cause a database collision, and will crash the program.
[marshall@likon] {21:21} [~] $ bitcoin-0.18.0/bin/bitcoin-qt
terminate called after throwing an instance of 'std::runtime_error' what(): BerkeleyBatch: Can't open database 8204388-wallet.dat (duplicates fileid 8b1c090000000800e25686350000000000000000 from 1352491-wallet.dat)
Aborted (core dumped)
I have been able to reproduce the issue by importing two of the same wallet (with different filenames).