You'll get the same pressure but the LPM will be linked. MY first jetter was 30LPM and you will have that across the two machines.
The difference when you jump up to 30LPM and above is immense but it takes twice the water, twice the fuel and twice the servicing.
It should be twice as fast, twice as clean and twice something else - just to keep it balanced but I cannot think of another twice
Rob