One transfer needle per product, no cross contamination, no pushing unconditioned air into your vials, very little purging necessary once you load your pen. This is my method for loading pen cartridges. I think it's less wasteful and more sterile than the "second needle for venting/breather" method.

My process:

Disinfect all the vial tops, pen vial filling station block thingy and whatnot that are involved in my current project
Use 3 ml transfer syringe to draw 3 ml of sterile air from empty pen vial (now it's under vacuum)
Use 3 ml transfer syringe to push 3 ml of sterile air into bac water vial
*DO NOT REMOVE NEEDLE
Draw desired amount of bac water out and remove needle from vial
Fill lyophilized product vial with bac water slowly down side of vial
While product is resting and rehydrating, use same transfer syringe to draw another 1-2 ml of sterile air from pen vial (stop if the rubber bottom plug starts to get sucked up the vial)
Once product is dissolved and gently swished for mixing, push the sterile air from your transfer syringe into product vial
*DO NOT REMOVE NEEDLE
Draw reconstituted product into transfer syringe
*If there's still too much vacuum in vial to draw out all of the product, invert vial and syringe, then:
Allow the air pocket in the syringe to be drawn back into the pep vial
Draw remainder of product out
Push the product into the pen vial gently down the side (be sure to have it on a flat surface in case it overpressures and tries to push the rubber bottom plug out of the pen vial)
Pull needle out until it's at the very top of the interior of the pen vial
*Start to draw air pocket out of pen vial with syringe
Once you feel a lot of resistance as you are pulling the syringe plunger, transfer pen vial to the bottom plug pusher part of the filling station block tool thingy
*Push the rubber bottom plug up as you are drawing the last of the air pocket form the top of the pen vial
*Pull needle out of rubber stopper as soon as it looks like the air pocket is gone, but before product starts going up into transfer syringe
*You're almost done, now purge your pen vial of air
*Put your pen vial into your pen
*Disinfect the top of the pen vial
*Put a pen needle on it
Cycle your pen plunger until it is touching the bottom rubber plug of the vial
*With your pen tip pointed skyward, "inject" 3 units at a time into the air until it shoots a stream up
*Now you really are done, try your concoction.

@STATIEEIGHT Everybody I've watched do the transfer to a pen vial uses the breather needle technique but I'm not a fan of it. I just use one of the pen vial filling blocks and use the single needle that I transfer the peps with.
Before I put my peps into the pen vial, I draw 3 ml of air from it so that it's under vacuum like the lyophilized vials always are.
I inject my peps into the pen vial slowly down the side then I pull the needle back up toward the butyl stopper and draw the air out of the pen vial with it as I push the pen vial down on the dowel that pushes the rubber plunger part of the pen vial up.
This way seems much simpler to me than involving a second needle. It's also more dummy proof. When you use a vent needle, you push product out of the vent needle if you push the rubber plunger up too hard.
I'm able to get the pen vial purged well enough that I usually only need to push out about 10 units of air through the pen needle tip to get the rest out. I could purge it more thoroughly, but I would rather not risk wasting product by pushing it up into the transfer syringe during purging.

One transfer needle, no pushing unconditioned air into your vials, very little purging necessary once you load your pen....

My process:

• Disinfect all the vial tops, pen vial filling station block thingy and whatnot that are involved in my current project
• Use 3 ml transfer syringe to draw 3 ml of sterile air from empty pen vial (now it's under vacuum)
• Use 3 ml transfer syringe to push 3 ml of sterile air into bac water vial
  *DO NOT REMOVE NEEDLE
• Draw desired amount of bac water out and remove needle from vial
• Fill lyophilized product vial with bac water slowly down side of vial
• While product is resting and rehydrating, use same transfer syringe to draw another 1-2 ml of sterile air from pen vial (stop if the rubber bottom plug starts to get sucked up the vial)
• Once product is dissolved and gently swished for mixing, push the sterile air from your transfer syringe into product vial
  *DO NOT REMOVE NEEDLE
• Draw reconstituted product into transfer syringe
  *If there's still too much vacuum in vial to draw out all of the product, invert vial and syringe, then:
  • Allow the air pocket in the syringe to be drawn back into the pep vial
  • Draw remainder of product out
• Push the product into the pen vial gently down the side (be sure to have it on a flat surface in case it overpressures and tries to push the rubber bottom plug out of the pen vial)
• Pull needle out until it's at the very top of the interior of the pen vial
  *Start to draw air pocket out of pen vial with syringe
• Once you feel a lot of resistance as you are pulling the syringe plunger, transfer pen vial to the bottom plug pusher part of the filling station block tool thingy
  *Push the rubber bottom plug up as you are drawing the last of the air pocket form the top of the pen vial
  *Pull needle out of rubber stopper as soon as it looks like the air pocket is gone, but before product starts going up into transfer syringe
  *You're almost done, now purge your pen vial of air
  *Put your pen vial into your pen
  *Disinfect the top of the pen vial
  *Put a pen needle on it
  • Cycle your pen plunger until it is touching the bottom rubber plug of the vial
    *With your pen tip pointed skyward, "inject" 3 units at a time into the air until it shoots a stream up
    *Now you really are done, try your concoction.