Optimizing Two Parallel IIR Filter Sweeps
Two (linear, time invariant) filters in parallel, $H_1(z)$ and $H_2(z)$, are equivalent to a single filter $H(z) = H_1(z) + H_2(z)$ (i.e. the sum of both transfer functions).

You don't give any information on how those filters are represented. Normally, they are represented in terms of the recursive ($a_k$) and non-recursive ($b_k$) coefficients (see Wikipedia for reference).

Now, if you are going to add both transfer functions, then you need to find the common denominator (which is the product of two polynomials, one from each filter), and more cumbersome yet is to find the new numerator polynomial coefficients.

If you are doing this automatically (since you adjust your filters periodically), I suggest you keep your algorithm as is.

Executing an IIR filter is not very CPU intensive since the order of the filters is normally not greater than 10 or so.

Note: if the filters were FIR, then you simply add the filter coefficients (there are no recursive coefficients (and no division of polynomials) to worry about). But FIR filters are normally much more CPU intensive than comparable IIR.