Just tested with latest free versions AUi and VSTi versions of TX16Wx.
I experience the same thing as you:
Crash with AUi version when choosing multiple outputs.
But VSTi version doesn't crash Reaper for me when choosing multiple outputs.
My guess is the issue has to do with faulty/buggy AUi version.
Reaper 5.96pre6
OSX 10.13.6
Macbook Retina mid 2015.
Snippet from crashlog:
Thread 0:: reaper Dispatch queue: com.apple.main-thread
0 libsystem_kernel.dylib 0x00007fff5bdbd20a mach_msg_trap + 10
1 libsystem_kernel.dylib 0x00007fff5bdbc724 mach_msg + 60
2 libxpc.dylib 0x00007fff5bfc5d6e xpc_pipe_routine + 302
3 libxpc.dylib 0x00007fff5bfc5aad _xpc_interface_routine + 167
4 libxpc.dylib 0x00007fff5bfc55d3 bootstrap_look_up3 + 185
5 libxpc.dylib 0x00007fff5bfc54ff bootstrap_look_up2 + 41
6 com.apple.DiskArbitration 0x00007fff35d45ce6 DASessionCreate + 146
7 com.apple.AppKit 0x00007fff31cbabe9 -[NSWorkspace getFileSystemInfoForPath:isRemovable:isWritable:is Unmountable:description:type:] + 319
8 com.cwitec.TX16Wx 0x0000000124a67029 0x124789000 + 3006505
9 com.cwitec.TX16Wx 0x0000000124801647 0x124789000 + 493127
10 com.cwitec.TX16Wx 0x00000001248012eb 0x124789000 + 492267
11 com.cwitec.TX16Wx 0x00000001247fc47b 0x124789000 + 472187
12 com.cwitec.TX16Wx 0x00000001247fb230 0x124789000 + 467504
13 com.cwitec.TX16Wx 0x0000000124886e58 0x124789000 + 1039960
14 com.cwitec.TX16Wx 0x0000000124a74913 0x124789000 + 3062035
15 com.cwitec.TX16Wx 0x0000000124885ed6 0x124789000 + 1035990
16 com.cwitec.TX16Wx 0x0000000124885f24 0x124789000 + 1036068
17 com.cwitec.TX16Wx 0x0000000124a7450c 0x124789000 + 3061004
18 com.cwitec.TX16Wx 0x0000000124a74554 0x124789000 + 3061076
19 com.cwitec.TX16Wx 0x0000000124a73877 0x124789000 + 3057783
20 com.cwitec.TX16Wx 0x0000000124a73ca0 0x124789000 + 3058848
21 com.cockos.reaper 0x00000001005203e1 SWELL_GetAudioUnitCocoaView(HWND__*, ComponentInstanceRecord*, AudioUnitCocoaViewInfo*, RECT*) + 305
22 com.cockos.reaper 0x000000010054ac9f AU_Plugin::ShowConfig(HWND__*, RECT*) + 895
23 com.cockos.reaper 0x00000001003e5a3c FxDsp::showConfig(HWND__*, RECT*, int) + 556
24 com.cockos.reaper 0x00000001003f97f6 FxChain:
nDspAdded(HWND__*, int, char const*, int) + 918
25 com.cockos.reaper 0x00000001004188ef addDialogProc(HWND__*, unsigned int, unsigned long, long) + 21151