Yes, what you are thinking is possible. I too was interested in making this build long ago and just now am i finally able to bring it to life. I have a dual wielding huntress that uses 2 riflecrossbows based on maximum attack speeds. Her name, "RapidFire"
You can use the offhand weapon, but i have not found a use for it with this character. Attacks are not as powerful as a huntress that wields a 2handed crossbow, but are much faster. It is up to the player's preference as to how to build the character (using dex or str or both). Strength will increase the damage value just like with a melee character, but only at half value, 50%. I personally went with strength because i have a good gear setup for accuracy. I only needed very little dex to get 200-250 accuracy so damage is more beneficial.
I also made a rampager with inverted skill selections to see which is better. While it is possible to make this build as a rampager, it is far better with a huntress. As a rampager you can only add 3 skills from huntress, usually fastreload, xbow mastery, and expertshot. But as a huntress you can benefit from the many more helpful skills that the class offers such as scatteredarrow and evasion. The hardest part about this build is that in order to utilize the nice attack speed bonuses, you MUST have dualwield skill added. This was a very difficult skill for me to find, but eventually it was possible
I use expertshot as a primary attack and scattered arrow as a secondary for mobs. The 3 extra skills i gave her were all from rampager. I gave her weaponmastery, motivation, and dualwield. So besides her two main attacks, there are also 2 buffs, fastreload and motivation. Motivation is very vital for huntresses because it adds damage as well as additional accuracy bonuses.
No, offhand weapon WILL NOT help with a 3rd velocity bonus. The only items that utilize attack speeds are the bows, armor, gloves, belt, shirt and artifacts. Trousers can also, but pantsofbravery work better for damage. Accuracy can be found on bows, armor, gloves, boots, belt, shirt, and artifacts.
The trick to this build is that even though you plan to use 2 bows, you will want MANDITORY high attack speeds on both bows but also dual leeching capabilities. You can sacrifice having the hp leech on a bow and use it on gloves instead. This might not be such a good sacrifice though if you plan to use both speed and accuracy on gloves (good luck getting all 3 stats on gloves lol)
I hope this helps out for now. Soon i plan to write another complete guide specifically for this build. This is a more complicated build to assemble so you might be better using a different build to hunt with for the time being and reserve this one as more of a "hobby" until you can find dualwield skillitem and all the other necessary equipment.