I haven't been able to find any definitive information on this issue. Obviously, the number of charges on a weapon enchantment is determined by the skill of the player, the size of the soul gem, the value of the enchantment effect, and the magnitude you choose on the slider (the charge / magnitude slider).

My question is this, how is the number of charges on a weapon enchantment determined when you use the "Extra Effect" perk to imbue the weapon with 2 enchants. For example, I made a bow with 1 enchantment (Soul Trap), and set the number of charges to the max, while making the magnitude 1 second of sould trap time. This gave me some amount of charges in the thousands on that bow. However, when I created a similar bow with an extra effect on it (Soul Trap with maximum charges + absorb stamina with maximum magnitude) the resulting bow only has about 40-70 charges. How does this occur? Does it matter which enchantment is used first? Is the resulting number of charges just an average between the two? Is there a way to make this bow have significantly more charges while maintaining the magnitude of the stamina absorption?

I would greatly appreciate any guidance here, as I cannot seem to get any info for this on the interwebs =P

Extra Effect is more better suited for Armor/Clothing if you ask me, was I think it's intention. If you do have 2 effects on a Weapon the charge does dwindle because of THAT simple fact. It makes perfect sense.

Also don't try and make a Weapon like say: 25 pts Drain Health, 25 pts Fire Damage. It'll cost alot compared to something like 15 pts Drain Health, 10 pts Fire Damage. Kinda hard for me to explain.. lol.

Thanks for the suggestions, but still looking for a more definitive answer on what the relation is between the two effects with differing charges.

As for using extra effect on armor rather than weapons, I have some pretty suitable uses for weapon use. Lets say you don't want to have to keep switching weapons to a soul trap enabled weapon in order to gain a soul to charge up your main fire damage weapon (which can get annoying), you can just make a fire damage weapon that also has soul trap effect, effectively removing an annoying step. Also, I am playing as an archer, and stamina is a huge factor when using "Steady Hand". By creating a bow that does elemental damage, AND absorbs stamina, I can effectively use steady hand to zoom in and slow time for as long as I need to, while consistently replenishing my stamina. Just some food for thought.

Well, you have only one soul gem with a fixed amount of power, but instead of only one effect that drains its power, you got 2. Each of the effects cost some of the charge,and they are always applied at the same moment. Furthermore, some effects require a larger amount of the charge then others. For further information, visit the following page:
http://uesp.net/wiki/Skyrim:Enchantments

If you use 2 effects on a weapon, each effect uses up the charge stored in the enchanted weapon. Absorb Stamina is a very expensive spell and uses up a lot of Magicka stored in the soul stone. Soul trap is very inexpensive spell on the other hand. So Absorb Stamina totally dominates in this case. Maximum Absorb Stamin will typically give you 70 charges with grand soul gem, and 1 second Soul Trap will give you 3000 charges or something like that. So doing both at same time use up charges from both effects and you get 70 or 69 charges. It's most noticeable if you use two expensive effects at same time, like Absorb Stamina and Turn Undead.

One way of increasing the number of charges on your enchanted weapon is to wear Fortify School of Magic enchanted clothing/armor appropriate for your enchanted weapon. Absorb Stamina is in Destruction school, so if you wear two pieces of armor or jewelry with 25% Fortify Destruction enchantment, you will get 140 charges. If you wear 3 pieces of clothing with Fortify Destruction enchantment, you will get 280 charges. If you wear 4 pieces of clothing with Fortify Destruction enchantment, you will get 3000 charges.

Thanks Raliel, I am still trying to wrap my head around how it is calculated, but very helpful explanation! But is the basic idea that, no matter how many charges each effect has, when put on the same weapon, the total amount of shots you can fire will be fewer than either of the charge numbers displayed for the individual effects? For example, soul trap for 500 charges, and absorb stamina for 200 charges, when put on the same weapon will amount to some amount of charges less than 200?