Fom looking at Inchworm's website it you order the Lefty, either already built or just the plate to build it yourself it make a passenger drop gear driven t-case into a driver drop t-case adn that is it. You would still need to purchase the plate adapter to mate your current transmission to the gear driven t-case. I will use Trail Gear 4.7 gears for this comparison as they are about $100 less in cost. Here would be the cost break down.
Pre-Built Lefty ($1,995), plate adapter ($350), 4.7 gears ($442), and Core Charge ($250) - $3,037
(May be more if they charge to install the gears, plus this price includes the core charge of $250 for a gear driven t-case that you either pay or pay to send a good unit, that you purchase locally, back to them to use on the next build. There is also shipping not included)
Dual case setup from Inchworm pre-built would require drive shafts and a crossmember. No need for 4.7 gears with this setup as you are approximately where you are with the geared Lefty.
Pre-Built dual case setup ($1,150), Trail Gear Tacoma Dual Crossmember ($348), and driveshafts ($400) - $1,898 (Again there would be shipping on top of this.)
Build it yourself and with friends, source a gear driven t-case would look like this.
Lefty Gear Driven t-case ($100), Lefty adapter ($1,124), plate adapter ($350), and 4.7 gears ($442) - $2,016 (Again there would be shipping on top of this.)
Dual Cases Gear Driven t-case ($100), plate adapter ($350), dual case adapter ($650), Trail Gear Tacoma Dual Crossmember ($348), and driveshafts ($400) - $1,848 (Again there would be shipping on top of this.)
Based on those numbers and for simplicity I would fork out the extra $50 and go with a pre-built case from Inchworm. This setup would give you the best gear selection and is much cheaper than a Lefty, pre-built or not.
You can also look at Trail Gear and Marlin for similar packages,
Trail Gear dual t-cases - Gear Driven t-case ($100), dual case adapter package ($566), Trail Gear Tacoma Dual Crossmember ($348), and driveshafts ($400) - $1,414 (Again there would be shipping on top of this.)
This option looks even better money wise, but you would have to put it together.
Marlin is currently out of stock and very proud of their parts.
As a side note, man am I glad I built a 1st gen 4 Runner, my duals cost me under $500 with the crossmember, even with 4.7 gears and twin stick still under a grand.
DD - 05 Toyota 4 Runner
Trail Rig - 88 4 Runner with dual cases w/ 4.7's, 37" PB Rockers and no lift
99' Toyota Tacoma Crushed by Toyota