Ну, вот так попробуйте
select q.f, p1.ss1 as ps1, p1.ss2 as ps2, p2.ss1 as ps3, p2.ss2 as ps4 from
(select iif(t1.dat>= t2.dat, t1.dat, t2.dat) as f from t1,t2 group by iif(t1.dat>= t2.dat, t1.dat, t2.dat)) q,
(select dat, sum(s1) as ss1, sum(s2) as ss2 from t1 group by dat) p1,
(select dat, sum(s1) as ss1, sum(s2) as ss2 from t2 group by dat) p2,
q left join p1 on q.f=p1.dat,
q left join p2 on q.f=p2.dat