My first thought when reading this verse is that when someone else is doubting God or that he/she seems to have lost faith, then another person who is not struggling at the moment--whose faith seems to be strong and not waivering- can be in prayer for them...can hold them up, so to speak.
I was thinking of my cousin whose health is not doing good right now and she is doubting God, her faith is weak. I can do a part by lifting her up, not just in prayer, but praying on her behalf.

Romans 15:1