Servo that have been modified for continuous rotation are no longer precision servos anymore, but rather they are morphed into becoming a geared bidirectional motor drive. And as there is no longer any internal feedback being used by the 'servo's' electronic one can't expect them to be exactly the same speed given the same angle command, even if the same model servo, unless you could match two from a large batch. Your best bet is to try and characterize your two servos and have a software 'offset' variable to adjust the faster to match the slower. However even then it might not be linear over the whole speed range.