You should not try use DMM to check the voltage on driver output to motors, because that is high frequency (range like 10's of Khz) while a digital multimeter reads like 3 times per second (3 Hz), so whatever the digital multimeter shows, the results ar not worthy of trust.
In conclusion, your motors just vibrate, your measured values are inconclusive. Because the motors vibrate in series, maybe one of them is skipping steps while the other doesnt, so they might fight each other sort of speaking. See that the motors are exact same type and same coil resistance & inductance, and also conductors wires are equal lenght to each coil. Maybe increase the current, or decrease it if needed. Turn them on and off and play with their angle, so their axial position will be both on full step when starting. Also if wired in series, the resistance increases (adds both coils) and if your coil resistance is like 6+ ohms, then in series will be double then the current wont have time to rise fast enough because the ohms law value is final value and that is just too close to the set point. So if the series connection doesnt work out because coil resistance is too much, then try parallel, but again preferably same wire length to both motors/coils.
In conclusion, your motors just vibrate, your measured values are inconclusive. Because the motors vibrate in series, maybe one of them is skipping steps while the other doesnt, so they might fight each other sort of speaking. See that the motors are exact same type and same coil resistance & inductance, and also conductors wires are equal lenght to each coil. Maybe increase the current, or decrease it if needed. Turn them on and off and play with their angle, so their axial position will be both on full step when starting. Also if wired in series, the resistance increases (adds both coils) and if your coil resistance is like 6+ ohms, then in series will be double then the current wont have time to rise fast enough because the ohms law value is final value and that is just too close to the set point. So if the series connection doesnt work out because coil resistance is too much, then try parallel, but again preferably same wire length to both motors/coils.