This paper reports an online reciprocal peer tutoring project for improving language competence in Spanish and English. Students aged 9-12 years from Scotland and Catalonia were matched to act as tutors in their own language and as tutees in a modern foreign language. Students were intended to improve both their first language (through helping the tutee) and a modern language (with their tutor's help). The methodology combined a quasi-experimental design and a qualitative analysis of texts. For Catalan students, pre-post test results indicated statistically significant improvements in reading comprehension (while acting as tutors) and writing (while acting as tutees). Scottish students improved only their writing (acting as tutees). Analysis of the texts showed that when more support was given, the tutor had more learning opportunities, but then there were fewer opportunities for the tutee, and vice versa. Thus the tutee learned more with less elaborated feedback, leading to fewer opportunities for tutor improvement. This paradox could be resolved by adjusting the scaffolding support given by tutors, to create a balanced interactive learning context for both members of the pair.