Both slow planets on one house
A house-matter fruits when BOTH Jupiter and Saturn simultaneously transit over, or aspect, that house (or its lord). Compute it as an intersection: list the houses Jupiter touches, list the houses Saturn touches, and the houses on BOTH lists are the double-transit set — the workhorse of transit prediction.
Marriage times classically to a double transit of Jupiter and Saturn on the 7th house or its lord.
Opportunity meets permanence
Jupiter supplies growth, blessing, and opportunity; Saturn supplies structure, commitment, and permanence. A lasting event needs both. When only Jupiter touches a house, the chance appears but may not set; when only Saturn does, structure forms without the opening. Together, on one house, they complete the event.
A job offer (Jupiter) that becomes a settled career (Saturn) is the pair working as one.
Worked example
Jupiter in Taurus, Saturn in Pisces, from Dev's Virgo Moon
Jupiter (seat + 5/7/9): Taurus is his 9th; its aspects Virgo, Scorpio, Capricorn are his 1st, 3rd, 5th — Jupiter touches 9, 1, 3, 5. Saturn (seat + 3/7/10): Pisces is his 7th; its aspects Taurus, Virgo, Sagittarius are his 9th, 1st, 4th — Saturn touches 7, 9, 1, 4. Both lists share the 1st and the 9th: a double transit on two houses.
Jupiter {9,1,3,5} and Saturn {7,9,1,4} intersect at the 1st and 9th from Dev's Moon.
Count aspects, and watch two targets
Both 'transit over' and 'aspect' count — that is why the seats alone (Jupiter's 9th, Saturn's 7th) can miss while the beams meet. And the convergence fruits a matter whether it lands on the HOUSE or on the house's LORD (module 4): a double transit on the 7th, or on the 7th lord wherever it sits, both time marriage. Check occupancy AND aspect, house AND lord.
Jupiter aspecting the 5th while Saturn transits it is a full double transit; so is both touching the 5th lord.
Window plus permission, over a real room
A double transit names the WINDOW; the dasha grants PERMISSION (n77); the chart supplies the PROMISE. The gold standard is triple confirmation — a natal promise, a dasha running the significator, and a double transit on the house or lord. Without promise and dasha, the convergence opens over an empty room and under-delivers; it raises the odds sharply, it never guarantees.
A 7th-lord dasha plus a double transit on the 7th, in a marriage-promising chart, is a strong marriage call; the same transit over a denied matter brings little.
The one idea to keep
Intersect the beams, then confirm the room
The double transit is Jupiter's touched houses intersected with Saturn's, from the Moon — opportunity meeting permanence on one house or lord — fruiting the matter when a natal promise and a running dasha back it. Window from the two slow hands, permission from the dasha, foundation from the chart.