I suggest you start designing the control of UPFC with designing the control of individual converters separately and then try to sync the control design based on your requirement.
I agree with Viqar. An effective control strategy is to control each power converter to aim it ac requirments, and use one of them to control the dc variable (voltage in a VSC based case) .
I recomend you these articles for UPQC case
Article Analysis and Design of a Multi-Cell Topology Based on Three-...
Article Analysis, Design and Control of a Unified Power-Quality Cond...
Article Design of a Discrete-Time Linear Control Strategy for a Multicell UPQC