import re
message = '<@U0104FGR7SL> test111 <@U0106LSJ> test33'
print ( re.split(r'\s+(?=<@U[^>]+>)', message) )
print ( [x.strip() for x in re.split(r'(?=<@U[^>]+>)', message) if x] )
aW1wb3J0IHJlCgptZXNzYWdlID0gJzxAVTAxMDRGR1I3U0w+IHRlc3QxMTEgPEBVMDEwNkxTSj4gdGVzdDMzJwpwcmludCAoIHJlLnNwbGl0KHInXHMrKD89PEBVW14+XSs+KScsIG1lc3NhZ2UpICkKcHJpbnQgKCBbeC5zdHJpcCgpIGZvciB4IGluIHJlLnNwbGl0KHInKD89PEBVW14+XSs+KScsIG1lc3NhZ2UpIGlmIHhdICk=