Multiboxing in Elite is extremely tedious...
I've tested this... and my brain hurts just thinking about it. And therefore I do not view multiboxing as an exploit.... here is why:
Combat is out of the question...
Trading only makes sense if you park your alt(s) in the starport where you sell and then concentrate on flying just one CMDR.
Rare Trading is probably the only activity where you really benefit, regarding time spent, at least when it comes to the "fill up" part. Then comes the travel... sigh (if you want your alt(s) to benefit from the dividends).
Wing Missions... You need tons of Luck in order to have your alt(s) pick up the same mission as your main... not worth the effort, tbh.
Mining only makes sense if you want to speed up a single CMDR to fill up faster, and you have to stick to Laser Mining, otherwise it is out of the question. But still takes a lot of time to fiddle with your alt(s) to move to the asteroid, mine, make sure the CMDR you boost have limpets running the whole time etc, etc... then comes the travel... for dividends... sigh
Nothing beats a wing with other people!!! For all of the above, get some friends and wing up. So much more fun as well.