I like the zip method suggested by @Netwave but you must have strings of the same size for it to work.
Here a patched version:
s1 = 'Hello'
s2 = 'Paul'
#make strings equal in length
if len(s1) != len(s2):
while len(s1) < len(s2):
s1 += ' '
while len(s2) < len(s1):
s2 += ' '
txt = "\n".join(f"{x}{y}" for x, y in zip(s1, s2))
print(txt)
Output:
HP
ea
lu
ll
o