You'd need someone who is both proficient with a non simple weapon that has parrying (quarterstaff it's the only simple weapon with parrying and it's two handed, shield has somatic component) and can cast first level spells. Not super hard to do but its a fringe stacking ability that at most gets a +9 to ac. A regular shield and the shield spell already gets to +7 and has been a part of O5e since it started without anyone complaining of a problem.
I'd honestly be stoked if my players did combine the two. Sounds like an awesome bladesinger type ability. "parry! Oh that only gets me to +2 and I needed +3... Shield!"