Yes. From the SRD:
You and another willing, corporeal, living creature of the same or smaller size fuse into one being. As the manifester, you control the actions of the fused being.
The manifester can continue to act, and retains their identity as "You".
The fused being knows all the powers you and the other creature know, has the sum of your and the other creature’s power points
You can manifest powers as normal with some augmentations, including this one.
There is no text forbidding double-fusion, whereas there is for stacking with fission:
You cannot use fission on a fused being.
Since all effects of fusion can be applied as-is to a manifester that is already fused, and since the manifester is rather different from when they first manifested fusion, double-fusion would seem to be a good candidate for exceptions to same-effect cancelling:
The same spell can sometimes produce varying effects if applied to the same recipient more than once. Usually the last spell in the series trumps the others. None of the previous spells are actually removed or dispelled, but their effects become irrelevant while the final spell in the series lasts.
It wouldn't make sense to avoid "removing" a previous manifestation of fusion if the effects are cancelled out — where would the previously merged creature go?