  Results are presented from a series of comparisons between two independent
femtosecond frequency comb systems at NPL, which were carried out in order to
assess their systematic uncertainty. Simultaneous measurements with the two
systems demonstrate agreement at the level of 5 x $10^{-18}$ when measuring an
optical frequency against a common microwave reference. When simultaneously
measuring the ratio of two optical frequencies, agreement at the 3 x $10^{-21}$
level is observed. The results represent the highest reported level of
agreement to date between Ti:sapphire and Er-doped femtosecond combs. The
limitations of the combs when operating in these two different manners are
discussed, including traceability to the SI second, which can be achieved with
an uncertainty below 1 x $10^{-16}$. The technical details presented underpin
recent absolute frequency measurements of the $^{88}$Sr$^+$ and $^{171}$Yb$^+$
optical clock transitions at NPL, as well as a frequency ratio measurement
between the two optical clock transitions in $^{171}$Yb$^+$.
