Software-implemented fault injection (SWIFI) is an established experimental technique to evaluate the robustness of software systems. While a large number of SWIFI frameworks exist, virtually all are based on a single-fault assumption, i.e., interactions of simultaneously occurring independent faults are not investigated. As software systems containing more than a single fault often are the norm than an exception [1] and current safety standards require the consideration of 'multi-point faults' [2], the validity of this single-fault assumption is at question for contemporary software systems. To address the issue and support simultaneous SWIFI (simFI), we analyze how independent faults can manifest in a generic software composition model and extend an existing SWIFI tool to support some characteristic simultaneous fault types. We implement three simultaneous fault models and demonstrate their utility in evaluating the robustness of the Windows CE kernel. Our findings indicate that simultaneous fault injections prove highly efficient in triggering robustness vulnerabilities. © 2013 IEEE.