Both my - bright - children go to a school that only does double (Combined) science GCSEs. It does, as a result, offer 5 free subject choices at GCSE, which is rare, so it is swings and roundabouts in many ways.
Interestingly, the school's A-level Science results are excellent, and very much in line with a similar school (in terms of demographics) that does offer triple.
Discussing it with various people, doing A-levels from the basis of double science where the school only offers double is pretty much as successful as doing A-levels from the basis of triple where the school usually offers triple - the teaching just starts from a slightly different point.
On the other hand, doing A-levels from a starting point of double in a school where most able scientists will have done triple is less successful, because the gap between double and triple is not always systematically filled.
I agree that it is somewhat unfair on those children who are e.g. exceptional at Physics but not at Biology or vice versa that their combined grade may not reflect their strongest subject (though it does disguise their weakest...) However, Double Science with its broad curriculum is still probably a better grounding in science than what was common in my day - that less able science students would do only 2 or 1 science subject and drop e.g. physics entirely at 14.