It's easy to link them together just assign the same TraderID.
I have something similar I use on one of my mods which maybe someone can play around with.
I have a craftable vending machine which anybody can craft, the TraderID is unique to a custom trader I created and uses a custom stock list - so whoever crafts this vending machine will see this same traders items.
I haven't looked around at the xml too much on this but looking in traders.xml
<trader_info id="3" reset_interval="-1" override_buy_markup="1.0" override_sell_markup="1.0" allow_sell="false" player_owned="true"/>
The 'player_owned' property might be the key?
Essentially you're looking for the property which allows players to add to that specific TraderID