Two single-precision versus one double-precision unit
The only thing that really needs to be duplicated is the adders in the exponent handling. The mantissa logic can be split easily for the basic operations, at minimal extra cost.

For fixed-point, you don't have adders to compare and adjust exponents, and you omit the barrel shifters for the mantissa (which are also easily split, so savings there are minimal).

Life gets complicated for trigonometric functions and denormals though. If you don't implement these, the extra cost for dual-mode FPUs should be minimal.