Second language acquisition (SLA) research offers valuable insight on how languages are learned and how they coexist and influence each other. Sign language learners offer unique perspectives on SLA, allowing researchers to test theories that are otherwise constrained by access to only one modality. Current literature on sign language learning focuses primarily on bimodal bilinguals, mostly hearing adults learning their first sign language (M2L2 learners). However, other groups of L2 signers exist, including deaf learners who have previously acquired a sign language and are learning a new one (M1L2 learners). M1L2 acquisition offers unique insights into complex interactions including multilingualism, modality, and timing of acquisition. We argue that M1L2 signers are a key comparison group for investigations of various L2 and so-called modality effects and also represent a crucial test case for re-examining the traditional constructs of "native speaker/signer" and the effects of initial language delay or deprivation on subsequent language acquisition.