After D-Day, did the USAAF have enough aircraft and crews that they could run a day and night campaign concurrently?
I think we could say the USAAF always had 'enough' a/c and crews to fly both day and night missions, but the day missions would have had to have been smaller than they were in actual history if the same command had pursued night missions at the same time. I don't see any reason or point in time where this would not have been true, ie there was never AFAIK any bottleneck besides available number of planes/crews in the process of conducting day raids that would allow the day campaign to be just as big if some of the a/c and crews were assigned to night missions instead.
That said, I'm not sure there were any insurmountable bottlenecks to having US and RAF a/c both operate at night without getting too much in one another's way, if it were coordinated.
But it seems the premise of the question is some good reason why the USAAF should have operated night raids as of ca. mid 1944. I don't see what this would be, given the basic belief that daylight raids were a viable way to degrade German war making potential. And while that might have been a highly questionable belief as it applied to the earlier smaller USAAF efforts ca 1943, it was becoming quite a reasonable belief right around mid 1944. The size of the effort had grown alot, it was clear that excessive losses could be avoided (something that *wasn't* clear wrt RAF night bombing in the same period, the German nightfighters were gaining the upper hand until the latter part of that year) and accuracy and target selection were growing more effective.
Although, to take a contrasting example, the USAAF did adopt a mixed day and night campaign against Japan with B-29's. The well known initial part of that story was (believed to be) ineffective daylight precision bombing in Nov 1944-Feb 1945 by the Marinas based B-29's (plus earlier pinprick raids, basically, by China based B-29's) followed by a series of night fire raids in March. But thereafter it was a mixture with some day and some night bombing raids, plus night mining raids, and towards the end of the war true night precision radar raids by 'Eagle' equipped B-29's, at least against high radar-contrast targets. There was day and night 'concurrently', but of course the day effort had to be smaller than if it would have been with all a/c flying day missions all the time.
It seems to me two key elements were absent in the ETO/MTO case compared to the B-29 case. First the USAAF didn't believe it was failing in day precision bombing ca. mid 1944 (and arguably was not, in fact arguably the B-29's weren't either, the Japanese a/c industry was actually seriously damaged in the latter part of the Nov-Feb B-29 raids, and damaged itself further by poorly executed plan to disperse its activities in response to the bombing). And secondly it didn't see a political/military rationale to adopt night area bombing v Germany (was there one? I guess we could reconsider this), and the USAAF lacked true night precision bombing at that time, though RAF possessed it to a limited degree, and USAAF would have it by mid-1945 over Japan.
Joe