I've just started this game again and an old question came to mind, do separate research missions work just as well as a single mission with multiple agents?

They work but there is not difference to time... having them each work on their own is best because they are harder for one enermy to stop/ kill and picking a planet with more buildings also appears to help i.e ship research on a planet with 4 ship yards is better than on a planet with only one shipyard.

Hmm I've read that differently elsewhere that a mission with two people takes longer to "report back" than a mission with one. Given that Research often has a chance to make no progress I've read recommendations that suggest only single-character Research missions to avoid taking too much time only to make No Progress.

Next time I play though ill be sure to only run multiple character Research as an informal test

In a sense you are both right. The key factors are what are the total number of research factors being applied. This is found under the agents status. IE Wedge Antilles ship research factors + Ackbar ship research factors + Adar Tallon ship research factors and the amount of ship yards on the system those individual agents are doing their research. For security I split them up and do that research where the most production facilities are. If you keep them together you run the chance of losing all 3 and going to the baseline research factor which takes forever to reach a successful mission completion. Doesn't matter if they are doing the mission as a group or individually except for the security and the amount of facilities being used which is the only thing that effects how long it takes to reach a successful mission outcome.