The second arrival, and why it sets the evening
On a trip where two partners fly in separately, the later of the two landing times is the only one an evening should ever be planned around.
The second arrival is the later of the two landing times on a reunion trip, and it is the figure that decides everything about the first evening. The earlier arrival tells you when the room becomes usable. The later one tells you when the trip starts.
The arithmetic
Count forward from the second landing rather than backwards from a wished-for hour. Forty minutes from the airport into the centre on a good run, longer with a checked bag. An hour that belongs to the two of you and should not be scheduled around. Whatever remains is the evening that is actually available.
The nine o'clock line
A rough rule from the arrangements written here: if the later flight is on the ground before nine in the evening, the first night can carry a booking. If it lands after nine, move the booking to the second evening rather than forcing it. An hour that starts at midnight with one person who has been awake since four is not the evening either of you was imagining.
Putting it in writing
Give the second arrival time in the first message, with the time zone spelled out, and give only that one. A landing time quoted as half past seven without a zone attached is a frequent reason an hour ends up agreed for the wrong part of an evening. The page on landing hours apart works the whole timing through.