Hi and Welcome to YT! Both of your babies are adorable.
Guinea Pig harnesses are very tiny- have you tried any cat harnesses? They usually fit small dogs well but aren't so tiny.
Another small harness that I like and use is:
Newegg.com - Coastal Lil Pals Harness and Leash Combo Blue 10-14
The Buddy Belt is a very nice harness, albeit a little pricey, but the smallest size is very tiny so that might be an option.
Another option is to check
Etsy - Your place to buy and sell all things handmade, vintage, and supplies for sellers who make custom harnesses and find someone to make one to your dogs exact measurements.
Good Luck finding something that works.