Sullivan model of the connected sum of two complex projective planes ($(\Lambda (a_2,b_2,r_3,s_3),dr=ab,ds=a^2-b^2)$)

The two quadratic relations form a regular sequence in $\mathbb{Q}[a,b]$, so their Koszul complex has only the quotient-ring cohomology. Represent the two degree-two classes by rational polynomial forms and choose primitives for the exact relations to obtain a quasi-isomorphism. There are no further generators.