Absolutely no problem with using two separate soft starters on the motors. I have done this many times.
If you set both starters up equally, then the start current on each will be similar, but if there is plenty of thermal capacity in the rotors of the motors, I would suggest looking at starting one motor only with a higher start current and then start the second when the machine is up to speed.
If starts are relatively frequent, you could add some logic and alternate the starts between the two motors.
The torque reduces by the square of the current reduction. If you start both motors at as 400% start current with a LRC of 900% and an LRT of 180%, then the start current is more than halved and the start torque is less than a quarter. 4/9 x 4/9 = 16/81 = 19.75% of the total start torque.
If we start one motor only at the same torque, the start current would be SQRT(19.75/90) x 900 = 596%
So this is 596% of one motor as opposed to 400% of two motors which is a much lower current.