This has been talked about in other threads but is still a good topic to talk about.
Go get the model numbers off the pumps and call Russ J. or Paul K and give them those numbers. Not all direct drive pumps will pull from a tank.
Since you are experiencing a loss of pressure with 2 machines off the faucet, if you went to a buffer tank, AND.......... your pumps are designed so they could pull from a buffer tank, THEN you could pull from a buffer tank no problem. No way of guessing at this, have the model numbers checked out so you will know.
Buffer tanks will help you in many ways.
With a buffer tank, you can get different unloaders or send the diagram to Russ or Paul and get their input on bypassing back to the tank so your pump will last longer and you will not experiencing the loss of pressure, right now with your machines the bypass is a short hose back into the suction line. With this setup you cannot let go of the trigger too long as the water will get hotter and hotter or turn off the machine so you don't damage the pump.
Start out with at least 50 or more gallons of water with the hose going and 2 machines will last a while but you will have to keep an eye on the tank to make sure the tank does not get too low.
Also with the buffer tank each time you start the machine, you will not experience the momentary cavitation like when you connect a garden hose to the machine and start it up, there will be some air in the pump/hose until the water is flowing where the buffer tank always connected to the machines.
Downside is if you want to move the machines around the house, you would have to change fittings to use the garden hose and the bypass hose would just dump onto the ground.
There are other benefits/disadvantages but I cannot think of all of them right now.
If you don't ever have to move the machines, AND after checking with the vendors that the pump will pull from a buffer tank, I would setup the tank.