Many families of natural products are synthesized by large multidomain biological machines commonly referred to as megasynthases. While the advance of mechanism-based tools has opened new windows into the structural features within the protein-protein interfaces guiding carrier protein dependent enzymes, there is an immediate need for tools that can be engaged to link co-translated domains in a site-selective manner. Now, the use of silylcyanohydrins is demonstrated in a two-step, two-site selective crosslinking for the trapping of carrier-protein interactions within megasynthases. This advance provides a new tool to trap intermediate states within multimodular systems, a key step toward understanding the specificities within fatty acid (FAS) and polyketide (PKS) synthases.