OK it does not appear there is a "twinning kit"designed for this unit.
However here is a solution that will work.
Each furnace has a transformer that provides low voltage to controls. Currently each furnace is independent.
So to control both units with one thermostat, we will remove the transformer from one unit and replace the other transformer with a larger 80VA transformer capable of controlling relays in both units.
I have made you a drawing explaining this.
From new larger transformer , run a common to all relays Diagram shows the fan relay.If you have other relays such as solenoid valves to open valves for heat or cool, they will need the same common from this new transformer.
Red (24 volt HOT) will go to R at thermostat.
Then from G (blower) Green will go to relay in both units.
This way on call for unit,we get voltage to relays in both units to bring on both units.
Let me know your thoughts and any questions