Looks as if your motors are high inductance types from the 1amp 5 volt rating. Your drive is of the current modulated type so it will sort out the current shaping (PWM) correctly for series connections of the two motors. For parallel IMHO you would not set the current as higher value as steppers run hot. You need to find out if your drives can be set for reduced current when not stepping - if they can then you could increase the current setting a bit but not to twice the current as you cannot be sure how the current will be divided. If you have the means of monitoring the current pulses in each motor winding then do so - lead lengths and lead routes should be similar for both motor to drive connections and to the power supply. In general parallel connection is to be preferred as it reduces the inductance seen by the driver and hence the speed with which the current can be made to rise in the time allowed by the pulse width modulated drive. Or put another way parallel connection is good for more torque and a bit more stepping speed.
There is a mechanical problem in that the drives need to be synchronised - this has two components - first the motor to axis drives should be mechanically connected at a stationary full step - secondly there is with all stepper motors an error in angular displacement with step command and this means that the two motors will tend to tussle for the position at each step and this might accentuate any resonance problems also there will be variations in the drive screws to consider.
Do I understand you correctly you are proposing to connect two lead screws one screw and one motor on opposite sides of a common gantry. If so the gantry will stall unless the screws and motor can free wheel when failed.
A simpler option might be to replace the motor with a more capable one.
Regards - Pat