Mutual interference between indoor adjacent wireless networks is becoming an important issue as it reduces the communication speed significantly. Isolating the wireless networks by using frequency-selective surfaces (FSSs) can be an efficient solution for such interference problems. As a solution proposal, a new periodic element geometry design, which stops incoming 2.4 and 5.8 GHz industrial, scientific, and medical radio (ISM) signals, is presented in this work. Mutual effect between each resonator (2.4 and 5.8 GHz) mostly leads to the optimization stage's being inadequate or time consuming at multiband FSS designs. Therefore, a new simple design technique is presented to be able to optimize each resonance frequency independent from the other. Besides, as an important feature, this technique also allows adjusting the frequency interval between the stopbands. Analysis of the proposed surfaces is executed with Ansoft HFSS v.15 software.