F is F A C
Am is A C E
Without a real bass note in the re-entrant C tuning, gCEA for chords. They will sound the same. They share two notes and are some times used a substitutes for each other.
I you are using the open position Am and F chords. Then try a different voicing, up the neck for one of the chords.
eg (string 4,3,2,1)
Am 2000
F 5553
of 3020 for F and 5452 for Am.
They will sound a little different.
Curt