MTTF(n-plex) Formula

Question

For an N-PLEX system, the formula is:

MTTF(N-plex) = MTTF(1-plex)^N/( N * MTTR^N-1)

However, given a system whose MTTF is 3 days and MTTR of 2 days... A 2-plex of the system would have MTTF(2-plex) = (3²) / (2 * 2^2-1) = 2.25 days.

That means the MTTF decreased by making it more redundant? What is wrong? Do I have the wrong formula?