you will normally need to change your master cylinder for a larger internal bore item if adding an extra caliper.
I'm not sure what size my 620 twin disc master cylinder is couldn't see any markings on it just now apart from brembo, maybe someone more knowlesgable will be able soon to help you out with an appropriate size required.
In the meantime this might help you if you know what size your caliper pistons are, you can then select an appropriate size master cylinder :
http://www.ducatimeccanica.com/brake_ratios.html
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