In this case, låtsa/låtsar vs låtsas, the passive form "låtsas" is used in the vast majority of cases. With the help of google i found more than a million hits for "låtsas" but no more than 20000 for "låtsar", although in the case "låtsar" is used, it seems to be interchangable. I wouldn't use anything but "låtsas/låtsades".