Hello SpeedyPC,
sandboxie works on-demand, i.e. he must specify which process will be executed in sandboxie (e.g. his Firefox browser) whilst autosandbox should sandbox usually only unknown/untrusted apps. There's a big probability that such autosandbox app won't be also run in sandboxie. If it does, it may not work correctly (sandboxing is quite complex in sandboxie/avast autosandbox and we never tested it together). If the sandboxed process is executed in sandboxie, or in avast autosandbox, there shouldn't be a conflict.
-pk