I need to hook 2 identical springs together in series and I am not really sure how to calculate the force.

I know how for one spring.
force equals initial tension plus rate times deflection.

If I hook together 2, does the force mulitply by 2? Is it the same?

Someone told me to use the following formula for calculating spring force for in series applications...
1/rate + 1/rate = 1/force

That doesn't make sense to me, maybe it's right, I don't know.

the spring I plan on using has the following specs...

free length 9"
rate 20 lbs/in
initial tension 24 lbs
suggested max deflection 3.2 "
suggested max load 87 lbs

the application required the deflection to be 2.5"

if I had two identical spring connected in series, both being stretched 2.5", what would my force be and how do I figure it out?