Depending upon which Engineering majors you are interested, UCI could either be a Reach or a Target school. UCI Math major would be a Target while Political Science major would be a Low Reach/High Target.
All Engineering majors at UCSD are Capped (Impacted) so UCSD Engineering would also be a Reach. Applied Math and Political Science would be in the High Target/Low Reach Category at UCSD.
Both UCI and UCSD admit into the University first, then into the major. The both consider alternate majors and recommend that the alternate major not be an Impacted/Capped major.
Much easier to switch out of an Engineering major than to switch into so keep that in mind. UC’s tend not to be flexible if you are not sure about which interests to pursue.