Welcome to the Forum Eric !
I think there is a concern joining the two alloys you mention.
While both base metals you list are deemed weldable you cannot possibly receive positive or negative guidance with the small amount of information you have provided.
This is a high level engineering decision that must account for many factors in design function, loads, fabrication and process control.
Even after the engineering decisions are made, it will be important to prove the procedures via procedure qualification testing in accordance with governing specifications or sound engineering practices.
Here is a link to a producer who may know of expert consultation services that you can get to help you move forward.
http://www.aircraftmaterials.com/data/aluminium/7020.htmlAlso... MIG and TIG wire are the exact same composition; they are just packaged differently
