I bought Lulzbot workhorse 1.5 month ago and I would like to buy dual extruder but im not sure is compatible to my workhorse and I would Like to know if I use flexi filament i need to buy aerostruder ?
I send email to lulzbot 1,5 month ago without answer maybe someone can help here 
I think you are asking two different questions…
#1 can you print flexible filaments?
Yes - that was part of the design of the Workhorse’s all-metal E3D Titan Aerostruder direct-drive head … it prints just about anything. Printing flexible filament isn’t a problem and you don’t need anything special. I’ve printed NinjaTek’s NinjaFlex and PolyMaker’s PolyFlex with no problems (although these materials tend to be stringy when printing but that seems to be typical regardless of printer.)
#2 can you install the LulzBot Dual Extruder head?
No – that head is designed to fit the TAZ 6 and/or TAZ 5. The mount designed was changed a bit on the new/current generation of LulzBot printers. Heads that fit the new Mini 2, TAZ Workhorse, and TAZ Pro have compatible mounting … but not the TAZ 6 or earlier.
There is currently no modular head that you can buy (ready to go) that upgrades the Workhorse to a dual-head design. So this would be a custom project.
I have wondered if it would be possible to build-up a TAZ Pro head (the design is online) to make your own. I haven’t tried.