I don't think it's that simple. My scenario is specifically a surprised and inappropriate loss of awareness LOA situation where you are below Rock Bottom gas supply but each diver has a personal supply adequate to complete a safe surface ascent. Do you donate beforehand or drain the tanks to near zero?
I think it is a good question and the appropriate response is situationally dependent and obviously somewhat subjective based on the donor's perception of the situation.
If you are sure both divers have enough to get to the surface safely and independently, then starting the process of sharing air is going to complicate the ascent and possibly delay it. In other words, if both have enough gas then they should never have to share air - even if one hits the surface with near zero.
If one diver is so low that it appears likely that an air share will have to occur sometime during the ascent (and the donor clearly has sufficiently supply) then I think it makes sense to start sharing air on the ascent and early in the process. If both divers are calm and capable, sharing air during the ascent should cause very little stress for either person.
If the LOA person appears very nervous, then the best solution is probably to begin the ascent immediately and get them as close to the surface as possible and then offer the gas when they seem calm enough to accept it.
Sharing early and saving air for BC and unforeseen emergencies on ascent makes sense, but if providing the air when deep is just going to compound the stress level, then it is probably better to avoid it until it is really necessary.
Also, the reality of the situation is that if a recreational diver ran the tank way down and is making a very low on air ascent and is (understandably) very nervous; when they get to 20 feet or so, they are probably NOT going to vent air, stop the ascent and do a safety stop. They are just going to go for the surface and be entirely uninterested in sharing air, even if it is offered.