If You put the machines in series, You would increase the voltage not the amperage. Series is like putting (2) 1.5 volt D cells in Your flashlight and using a 3 volt bulb. +from one battery connects to - on the other battery, the remaining + & - goes to the bulb.
Putting them in parallel will keep the voltage the same but add the amperage of the machines together, this is what You want to do.
Parallel is the way (2) 12 volt batteries are used in a truck with a 12 volt system.
The + from both batteries or welders are connected, the - from both batteries are connected, and the paired batteries or welders feed the truck, or welding operation.
Old machines or new machines, it will always be the same.