Summation of two IIR filters
In theory you just do it the same way. The transfer function of a weighted sum of filters with weights $w_i$ is just
$$H_{total}(z)=\sum_iw_iH_i(z)\tag{1}$$
where $H_i(z)$ are the individual transfer functions.
However, in general the order of the total transfer function is the sum of the orders of the individual filters, unless some filters share the same poles. Consequently, the resulting filter order can become too large for a practical implementation. If the orders of the individual filters &quot;are generally around dozens&quot; then you have a problem, even using floating point arithmetic.