Hey SergeyKRD,
Thanks for reaching back out. We just want to make sure we're on the same page. Just to confirm, are these two different documents or is it different pages in the same document? When you click the plus icon to "add" the signature after the first one, does the option not appear to add the signature for another document? Make sure these are the steps followed to add signatures to a document: Use Markup on your iPhone, iPad, or iPod touch - Apple Support.
If you are wanting to add signatures to several locations at once or across several documents with a single tap, this is currently not a feature, but you can suggest what you'd like to see regarding that here: Product Feedback - Apple.
Take care.