If you try to use both of them, you'll get an "Invalid argument" error.  For example, if you pass --wiredTigerEngineConfigString cache_size=64M --queryableBackupMode, mongo will end up passing the string ...,cache_size=64Mreadonly=true, to WiredTiger.

The bug is at https://github.com/mongodb/mongo/blob/b7a866da63617afe5bbb98d13b905fab63449209/src/mongo/db/storage/wiredtiger/wiredtiger_kv_engine.cpp#L513; that should be ss << ",readonly=true,"; (note the addition of the leading comma).