Don't put anything in the path of the outlet of a dryer, the reason is lint will catch on it and build up which can lead to a fire. What you want to do is join the two lines so that the airflow is like a Y so that air from each dryer will flow outward to a single pipe. You can find the fittings in the HVAC area of stores. Air likes to take the easiest path and going with the flow of a Y fitting makes it where the air doesn't want to go through the Y then back towards the other dryer. The Y fitting needs to be as close to the exit as possible.
In a commercial laundromat where multiple gas dryers exhaust into one much larger duct there are "check-vales" in the form of metal dampers on each machine that prevent above-described backflow. They don't always work well.
Of course residential style machines don't have one.
As a reminder, clothes dryers should NEVER be exhausted into the ductwork of any other type of appliance.
Stove/cooker exhaust hood: GREASE and LINT combined is a big fire and safety hazard.
Hot water heater/furnace boiler. JUST DON'T. The dryer will force poisonous gasses back into the living space by way of these appliances which add their own poisonous byproduct
Bathroom exhaust fans: Can't run both a the same time due to excessive volume of air.
In my opinion just install another separate vent. Relatively easiest and safest thing to do.