Connecting 2 excel sheets on Sharepoint

Hi all, 

I am new to excel, and I am trying to separate 2 sheets from 1 excel file because it is making the file tooo slow. 

But after splitting them, I don't even know how to connect them again :,( I created them on googlesheets, it was frustrating enough to know it does not convert to excel as it was, and now splitting it seems even harder. 

Does anyone here could help me with this issue? like what sort of formulas would work to connect both sheets but not making the each files extremely slow and big because, optimally, I would like people could use it (to input) on web.