How to transfer data from 2 forms on the site to one Google Spreadsheet?

Question

Google

Site of 3 pages
1 page - form 1, the data from the completed form is transferred to the Google table
2 page - form 2, the data is transferred to the same table
3 page - thank you for filling
out what is the problem:
1. when one user fills out the first page, the data is stored in the first row in the table, when the second page is filled, the data is transferred to the second row of the table - it is necessary that everything be transferred to one
2. due to the fact that the data is transferred to different lines, it turns out that if 2 people start filling out at the same time, then their data is mixed
user 1 > page 1 > row 1
user 2 > page 1 > row 2
user 2 > page 2 > row 3
user 1 > page 2 > row 4

Answer 1

Send data to the table when it is fully formed, not in pieces. Or look in api how to get the line number in which the data is entered, and edit (update) this line in the second step

Answer 2

pass in two tables
only xs if someone has not filled in))) an empty line upon filling in the first