It seems like the different rule sets have different requirements. Could someone make up a table so we can more easily see which rules apply for each official rule set? Mixing and matching them obviously won't work unless you are using your own custom setting with custom rules.
We can try. I can try.
GENERALLY, the prebuilt iconic starships for all of the following rulesets have comparable crew requirements (plus or minus). The crew requirement rules for CT, GURPS, Mongoose, and T5 are similar, while the crew requirement rules for MT, TNE, and T4 are very similar. MT's rules are heavier formulas than CT, and I think are calibrated to more or less work with small starships as well as huge ones.
LBB2 1977 p16
One person may fill two crew positions, providing he has expertise to otherwise allow him to perform the work. However, because of the added burden placed upon him, he is unable to apply his expertise to the position (that is to say, he is not allowed expertise DMs in either position).
LBB2 1981 p16
One person may fill two crew positions, providing he or she has the skill to otherwise perform the work. However, because of the added burden, each position is filled with skill minus one [...]; thus, to fill two positions, the character must have at least skill level-2 in each (except steward: level-1).
TTB p61
One person may fill two crew positions, providing he or she has the skills needed for both jobs. However, because of the added burden, each position is filled with skill minus one [...]; thus, to fill two positions, the character must have skill level 2 in each, except steward, which requires level 1. No person may assume the duties of more than two crew positions except in the case of an emergency.
Starter Traveller, page 33.
One person may fill two crew positions, if he or she has the skills needed for both jobs. However, because of the added burden, each position is filled with skill minus one [...]. Thus, to fill two positions, the character must have skill level 2 in each, except steward, which requires level 1. No person may assume the duties of more than two crew positions except in the case of an emergency.
MegaTraveller Imperial Encyclopedia, p90
I didn't find any obvious rules for running two skills at the same time, although there's likely a mechanic for it.
TNE, p222
No obvious rule about crew running two positions. I'm sure there's something in there somewhere.
Traveller 4th
No obvious rule about crew running two positions.
GURPS (Classic)
Page 171: Smaller vessels often have the same crewman performing multiple roles. If a crew member is filling multiple positions during a particular space combat round, he will suffer a -2 penalty on all skill rolls for each extra task he is performing. [...] A ship with more than two crew engaged in the above tasks should also have one captain who does nothing but give orders – if not, the GM can apply an extra -1 penalty to everyone’s skill rolls!
Traveller D20
I can't find anything obvious.
Mongoose 2e
Ships of less than 1,000 tons can, in theory, be run by just one or two multi-skilled people but the ship will be at a serious disadvantage in high-stress situations such as combat. The crewman acting as pilot will likely be kept busy in the cockpit, actually flying the ship, while the other will find their attention split between engineering, damage control and the weapon systems – a quick look at the Crew Duties section ... will quickly demonstrate how inefficient this will prove to be.
(The Crew Duties section is fairly standard Traveller for itemizing crew requirements)
Traveller 5th Edition (2018)
p61.
Crew Structure Can Be Adapted For Ship Size. On small ships, some crew members occupy two positions: pilot/astrogator, engineer/gunner, sensor tech/steward. Many positions are omitted or consolidated. With automation, a ship crew can be as small as one person.