I would fax them separately with a cover letter something like:

Attached is for 2018, there will be a 2019 sent shortly. Please try to process together. 

then 

Attached is for 2019, there will be a 2018 was sent right before this. Please try to process together. 

I am always hesitant to put two things in the same "envelope" for fear that they get stapled together and only one gets processed.