I think that the TIU needs to be V 5.0 or later. So, you may need to update an older TIU. The DB9 USB to serial converter is cheap on eBay. I also purchased a "1FT DB9 Y Splitter Cable Video Adapter Male to 2x Female PC 9Pin D-Sub RS232 12"". This allowed me to connect my TIU to the WIU AND a TMCC Command base with MTH 50-1032 cable.
Theoretically, you just may be able to connect two TIUs and the WIU with a USB Hub. There is a 4 port version on eBay for $4. This would be worth trying before you buy two WIUs. If both TIUs are set to TIU #1, it ought to work. I know you can talk to two TIU #1s using one DCS Remote. You may get lucky and it just might work the same way with WIU. If it doesn't work, no harm, no foul. Toss the hub in your computer bag. FWIW just for grins, I just bought one. I'll be testing it on RC3R's TIUs and WIU in the near future.
I did just receive and test a "5.5x2.1mm 12V CCTV Camera 1 Female to 2 Male DC Power Splitter Cable Adapter". You can power the auxiliary ports for two TIUs with one Z500 brick.