Same speed assuming the internal slippage is the same. If one motor is loaded heavily and the other lightly then the internal slipage could cause slightly different speeds. But if load is similar and condition of motors is equal (inter slippage same) then they will run the same speed. If you want them to run different speeds simply change displacement appropriately. A motor of twice the displacement will turn half as fast. Direct correlation.

One problem you can run into with motors running serial if you are turning them slow speeds especially and connected with a fair amount of rubber hose is one of the two can get to "chattering" or pulsating. A harmonic can set up and the expansion of the hose allows a slight accumulator effect and one motor will vibrate and chatter if it is driving a chain. Don't ask me how I know. Running at higher flow rates and speeds not likely to be a problem.

If running in series the first motor has to be designed for power beyond. If it is not can blow the seal. Eaton has a web site with all their motor specs as far as what they will tolerate for pressure, back pressure, displacement, rpms at flow, etc. I'm sure other companies do also.

John