Hi,

Two things:

1. you need make a common unit cell with equal lattice parameters. In your case you have 3.98 for Pt and 4.15 for Ag. So, you either have to compress Ag until cell to 3.98 or you expand Pt unit cell to 4.15, this way you can put both the Pt and Ag in one common unit cell.

2. After you create individual cells, you can put them back on top of each other using VESTA software.

I think this explanation is bit Raw, you will a bit more details. You can look into this video on Youtube, link below:

https://www.youtube.com/watch?v=-HGpX1snrlE&t=83s

I hope this helps!

Thanks,

Khagesh