There are a number of things to take into account:
- The distance between the runways; can they be operated as independent runways? This requires the centrelines to be more than 1035 meter apart.
- The traffic mix: is there constantly the same distance between landings or a large gap every now and then between a Heavy and a Medium that allows for one or two departures in between?
- The runway entry and exit points. Can aircraft wait sufficiently short of the approach end to allow for a rapid departure when cleared for take-off? Does the runway have high-speed exit taxi-ways at appropriate places for the landing traffic to vacate the runway quickly or is a slow 90 degree turn required?
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.