It can be done with them in series.  There isn't a lot of reason to do so, but it will work fine as long as both motors are sized the same and in good condition.   The only reason I can think of for doing it is to distribute the drive force to two locations, such as both ends of a shaft, or two drive pinions on a single sun gear.  The physics of it will balance the pressures between the two motors, so that for instance with 3000 psi input to the drive system, the line between the two motors will be at 1500 psi.  The first motor will likely require a case drain to operate with that amount of back pressure. 

When we got our litter spreader it had two motors mounted to the NewLeader gear box and plumbed in series in this manner.   The motors were 2x larger then specified for it, and I believe they had been put in series to  return to standard speed after having been upsized to provide greater torque for different conditions.   We put them back in parallell, then later installed one of the standard motors to increase speed half way to what it would be with the standard motors while retaining a 50% torque increase.