Does the Principle of Double Effect Work for Infertility?

Yes, indeed it does.

If one of a married couple is infertile or becomes infertile, the principle of double effect comes into operation and holds theological water, in stark contrast to condoms.

This is because while the outcome of sexual intercourse is known not to result in new life due to one partner lacking fecundity, no practical attempt has been made, either through intention or application, through the employment of a barrier between spouses, to thwart the creation of new life or to restrict or inhibit total self-giving in marriage.

It is not the sin of the couple that means that no new life will come about, because they are fulfilling the unitive purpose of sexual intercourse within marriage.

St Gerard Majella: Patron of the infertile

Furthermore, the couple do not defy God's sovereign power or the supernatural gifts of God, in as much as history bears witness to the power of the intercession of the Saints in granting, by their prayers, to a number of couples, the number of which is not negligible, who piously adhere to popular devotions, encouraged by Holy Mother Church, that which is impossible for man, because such gifts are both possible and easy for God.

For just as condoms are not 100% effective and have a failure rate, the number of which is rather conveniently unquantifiable, so it is true to say that many couples have found that either one or the other is not 100% infertile. Therefore, they are still, despite diagnosis, in principle, open to the possibility of new life.