I origanally posted this in the wow bug forums but it got deleted, so now ill post it here.
It's not so much an exploit as it only works for 1 rank, but it does save 2 days of artifact research, and only after you have boosted a character to Artifact knowledge level 20.
Once you boost a character to artifact knowledge level 20 and put in your next research note, it has a 2 day timer
If you then start researching another one, the total timer goes up to 7 days.
so adding the second research note, extends the first one by 2 days.
Example
boost to 20
put in order for 1 artifact knowledge = 2 days
put in another order the first one then takes 4 days to complete, and the second one comes 3 days after.
I know this because i had 6 hours left on my first one, then decided to start my next one and the first one had 2 days of time added to it.
I used another character to test this, and put in 2 orders, total time 7 days.
I used my last character to put in 1, and got it after 2 days
Once i collected that research note I put in my next order, which only took 3 days.
So my last character got to artifact knowledge level 22 two days before my 3rd character, who started 10 minutes before him.
Sorry for a poor explanation.