Double boxing is common variation of multi boxing concept, widely used in MMO like WoW.
Principle is straight. You use more than one account to achieve some goals. It can be mining, manufacturing, PvP or whatever in EvE. Many people use double boxing for running missions, scouting, solo PvP, u name it. Practically it is running 2 or more EvE clients simultaneously on the same computer in different windows, sometimes on 2 or even more monitors if your PC hardware and performance allow this.
Does it violate EvE EULA. Sims like no, unless you don't use macros or bots. As far as your character requires your intervention in control it is Ok. I am not 100% sure what is CCP policy on semi afk tankers, but it looks like ok. If you get banned for afk tanking, please do not blame me for such advice on tactic.
Therefore CCP has some limitation on multiboxing. You can't run 2 instances of EvE on one computer unless both have subscription. In another world you use trial account it is only one you can run on your PC. Of course you can run second trial account on notebook or second PC. Does it violate EULA? Honestly, I hope no.
By using 2 ships on different account in 2 different windows of 2 computers allow you to split agregating damage ( tanking) and elliminating rats as 2 different task. This way one ship can be fitted for tank entirely, and the second one for damage dealing.
Personally, I prefer and use 2 paid accounts on one PC in 2 windows. I used to use 2 computers when I just started play EvE tho. Playing on one PC with one keyboard and mouse is just way too convenient compare to 2 PCs. Even with one monitor.
Many hard core players may say it is lame in noobish and it is partially true. But this tacktic is undeniably effective and do not require month of training and tons of skill points invested. Most of the people who say double boxing is dumb, use second accounts for scouting themselves, so it is very biased opinion in most cases. Well, I can run 6/10 in my Pilgrim. Is it effective compare double boxing. Of course not. And it required like 6 month of training to be able to fit and fly recon ship at assessable level.
From another hand, all those Tengu pilots, including myself some times, use overpowered ships for hi sec plexing. It is even more lame than use creative approaches with only 1 week old chars.
If we are talking about radars in hi sec, you can use just one char and you wil be fine. But if you are going to run combat plexes like GSO, or go to low sec there no much options for beginner pilot.
While I still have commitment to make 300 mils in radars in 14 trial days, and I will do so, I would like to run some plexes as well, as far as I find them on the go and go to low sec ( what CCP wants :-) ) to find and run 4/10 GSO. I know, it is gambling, but I like it, especially not much I can write about radars atm.
I have 150 mils in my wallet, so it gives me a bit of room for maneuvering in terms of ship and fitting.
So, lets look at speed tank for Guristas Scout Outpost DED rated 4/10 plex, especially I have found few so far already.