Need help with babby booties pattern

Pattern Central
1

Here is the pattern and I am completely stuck on row 25 for the right strap, I don’t understand if I need to use a second strand of yarn and if I do why. Couldn’t I just keep going with the strand I have.

It says co 10 using longtail caston and second strand as a thunmb strand.

I have no idea what using a second strand as a thumb strand means.

Please help me someone!!

http://members.home.nl/tdpj/Patronen/Bootees/Saartjes%20bootees.pdf

2

Never mind I found other people who had the same problem with the same pattern and figured out what to do thank you anyway.