Both batteries are not an absolute must,but its better to have both,you'll need full cranking power to fire up a dead 6.2 thats sat awhile and likely has lost its prime to the injectors..
If you try using one battery,the passenger side one is where you'll put it--and you must either dissconnect the other positive cable going to the drivers side battery,or put something over it so it cannot touch ground and fry wires..
There are lots of ways to bleed the air from the injector pump and lines & injectors..one way is to loosen a few injector lines and crank it over--taking the glow plugs out helps it spin over faster and prime quicker,but those can present problems of their own..usually you'll kill the batteries by the time its primed enough to start,and they'll have to be recharged before it'll spin fast enough to fire up..
One "new" way I've tried to prime a diesel that was apart and the fuel system was "dry" and air bound,was to take a vaccum hose off a running gas engined vehicle and put it on the nipple on the injector pump where the return line was...
I made a "bleeder jar" to bleed brakes this way and it works well to bleed brakes,and also as well to bleed diesel fuel systems..
--all you need is a glass or thick plastic jar with a metal lid preferably,poke 2 holes in the lid and insert 2 pieces of 3/16" brake tubing in the holes,one almost to the bottom of the jar,the other only in about 1" under the lid..seal up around the lid where the tubes go in with solder or J-B putty..it needs to be airtight..you'll also need enough 3/16" vacuum hose to reach from your injector pump to the vehicle providing the vacuum..about 10 feet or so is good--I used cheap clear vinyl hose from a hardware store that was like 15 cents a foot,it works fine..also lets you see the air bubbles..
To use it,hook the vacuum hose coming from the gas engine of another vehicle ,to the shorter piece of tubing on the jar lid,the other length of vacuum hose from the jar goes to the injector pump return line nipple.
--I use a vise grip to pinch off the hose for an "on-off" switch to control the vacuum source..you start the other engine,open the vise grips,and it might take several seconds,but you'll see diesel get sucked into the jar--let it get sucked in until you see no more bubbles..you may need to stop and empty the jar,but if you used a good sized jar you probably wont have too (about 1 qt is usually plenty)..
Then it should start with a minumum of cranking,provided you get the glow plugs to heat up by by-passing the relay..you can do it the way Rene suggested,or simply jump both of the large studs with the thick red wires on the glow plug relay together with a jumper cable for about 10 seconds..
Let the starter cool off for at lest 3-5 minutes for every minute of cranking or it'll melt..dont ask how I know that..
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--also resist the temptation to "just use starting fluid"...it might get it to start,it might also bend a rod or damage a piston too...
