Leash training two puppies at once

Question
You were very descriptive regarding the type of collar, but do you by any chance have a photo sample you can share?  I feel this is the key, so I want to get it right.  Also, if I train these two separately, will it be obvious when we can all walk together?

Thanks again, I am encouraged again!
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The text above is a follow-up to ...

-----Question-----
My puppies are 6 months old now and I have been attempting to socialize them.  This includes two walks per day plus extra/longer walks on the weekend.  I have a male and a female mixed breed.  They are Australian Shepherd/Heeler/Boxer mix and weigh about 30 lbs. each.  Their problems are the following:
1.  Pulling on their leash, I have tried a couple of techniques to correct this, but two together is tough.   They seem to do better alone, but I would love to have them together;
2.  My male is showing aggression towards other dogs and sometimes people.  I find myself actively avoiding these "obstacles" while walking ie. scanning ahead and avoiding other dogs before the encounter.  I know this is not the right or best solution.

I would welcome any tips/tricks you may have.  I will be training them alone, and you can assume there will be no assistance.

Thank you for your time and expertise.

Kona and Ramesses thank you as well!
-----Answer-----
Yes, plan on working them separately.  They know your are at a disadvantage when you try 2 at a time, and run over you.  

The key to most behavior problems is approaching things using the dog's natural instincts.  Dogs see all the people and dogs in the household as a pack with each having their own rank in the pack and a top dog.  Life is much easier if the 2 legged pack members outrank the 4 legged ones.  You can learn to play the role of top dog by reading some books or going to a good obedience class. A good obedience class or book is about you being top dog, not about rewarding standard commands with a treat. Start at http://www.dogsbestfriend.com/  For more on being top dog, see http://www.dogbreedinfo.com./topdogrules.htm

Your puppies are old enough that you can start with a good 6' leather
leash and a sturdy slip collar, the metal chain ones with the rings on each
end. You want the shortest one that will go on and off easily. If you walk
with the dog on the left, pull the chain through one loop forming a "P".
Facing it, slip it over its head. The free end should come over the neck to the leash, and the other end should drop slack when there is no pull on the leash.  

Easier dogs will give up their pulling with a few good snaps of the leash
combined with a stern "Bad dog!".  You can work up to forceful corrections
with the leash doubled up in both hands and your whole body behind it. But
you don't want to use any more force than you need.  One gentle technique I
like is to just stop when he pulls.  He wants to go.  If you move forward when
the leash is slack, and stop when he pulls, he should quickly figure out the
only way to get to go, is not to pull.  This is about teaching him not to
pull, not getting somewhere.  The man that taught it to me said "If in a half
hour you haven't made it out to the front walk, fine, you have taught him a
lesson.

Still, you may want to switch to a head collar.  The leading brands are Promise,  Haltie, and Gentle Leader.  They have a strap going around the dogs nose looking something like a muzzle.  They work by pulling the dogs head around.  No other way gives you such great control with so little force.  The prong collar is now a dangerous relic of value only for its macho looks.  Do not consider using one without hands on instruction from somebody with plenty of experience with them.

On thing to do is to anticipate problems and work at keeping his attention on you as another person or dog approaches.  If he is focused on you, he can't show aggression to the other dog.  Besides, if you are top dog, then the other dogs are your problem.

Answer
I have some pictures on a web host, but none of them show a Gentle Leader  http://www.photolocker.net/images/Labman/breedsofdogguidepuppies.jpg  I did a search and found some,  http://search.creativecommons.org/?q=%22Gentle+Leader%22&sourceid=Mozilla-search

I can't say even after you have them heeling nicely individually when you can do 2 at once.  Maybe when they are doing well off lead.