Assuming taxiing is not a consideration, in an airport with two parallel runways does the airport gain capacity by:
Would there be other ways to divide the movements (say by class/size of aircraft)?
There are a number of things to take into account:
In general, mixed mode operations can increase capacity significantly. For example the estimate for London Heathrow is that the traffic could increase from 480.000 to 540.000 movements.
But it is not a simple change. The requirements on ATC increase. For example, to monitor the parallel approaches, the update rate and accuracy of the surveillance system must meet higher standards than for single runway approaches. Also missed approach and emergency procedures need to be changed.
And then there is an important noise issue; in mixed mode operations, all four runway ends are continuously exposed to aircraft noise, while in dedicated operations the departure end of one runway and the arrival end of the other is not exposed.
"Back of the envelope" estimation:
For simplicity let's assume traffic that is only Medium wake category with 3NM final approach separation at about 150 knots groundspeed. This would result in ~75 seconds intervals between aircraft passing the threshold for a pure landing runway, allowing for 48 landing per hour.
In mixed mode, a line up and hold instruction would be given as soon as the landing aircraft is passed the numbers, so take-off will commence quickly after take-off clearance is given. The take-off clearance itself can be given as soon as the landing aircraft has vacated the runway, which should be about 60 seconds after it crossed the threshold. The take-off run itself is 30-40 seconds for B737 / A320 family aircraft. Let's add about 20 seconds reaction and engine spool up time + margin. The next landing aircraft can cross the threshold after lift-off. The interval would be ~ 120 seconds between landing aircraft, resulting in 30 landings per hour. But this can be done on two runways at the same time, effectively increasing the throughput by about 25% percent.
The above schedule is very idealised with pure alternating take-off and landings and little margin for error. If put in practice it will probably result in too many go arounds due to late take-offs, but it gives good idea of the capacity increase that is possible.
That would no doubt depend on a lot of factors. Geometry for example. If the parallels are far enough apart there's no chance of a missed approach on one interfering with traffic on the other, you're a lot more flexible than when they're close together.
Maybe you have noise abatement procedures that make the use of one of the runways for either landing or departures illegal at specific times (e.g. Schiphol can't use certain approaches during the morning rush because overflight of parts of the city are not allowed at those times).
Type of traffic is another. If you're getting a lot of inbound heavies at an hour where you have a lot of outgoing smaller aircraft, it soon becomes a good idea to have one runway set aside during that period for landings, one for arrivals (the wake turbulence of the heavies would mean longer waiting times on the runway for the lighter aircraft).
So overall, there's no definite answer. It all depends on traffic, local conditions, etc. etc.
