Niʿma is a fixed past-form verb used in praise. Al-sayyidāni is its nominative dual subject, signalled by the alif ending. The people receiving the praise are the two leaders addressed in the following wujidtumā.
In wujidtumā, the attached second-person dual element functions as the deputy subject of a passive verb, rather than an ordinary active subject. Together the two constructions move from declaring praise to addressing those who deserve it.